The Cottage Smallholder

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Cooking in a Thermos or vacuum flask. Leek and potato soup recipe

The raw ingredients

The raw ingredients

When I first heard about cooking in a vacuum flask I had an image of a whiskery maiden aunts filling vacuum flasks with Spam, gravy granules and boiling water.

Why on earth would I want to cook in a Thermos?

The idea seemed grim until I bought my 1920’s Thermos at the village fete one year. This is a marvellous piece of kit. It was crying out to be used so I started researching the possibilities and I’ve used it for slow cooking ever since.

During WW2 vacuum flask cooking was normal. Apart from a few minutes coming to boil on the hob and a kettle of boiling water, it uses no energy. Like slow cooking and now, halogen cooking, seals in and enhances the flavours. So why not use cutting edge technology that was developed at the start of the last century to cut your energy bills and produce great food?

Last Sunday we were thrilled to be included in an article by Sarah Lonsdale on frugal cooking. This appeared in The Sunday Telegraph. It featured our 1929 Thermos – which has a capacity of 4.8 litres.

This got us thinking. Wouldn’t it be fun to develop a series of recipes for vacuum flask cooking. As most people don’t have access to a 4.8 litre flask we decided to invest in a good 1 litre flask. A flask that would keep water and food hot for 24 hours. The Thermos brand claims that its flasks keep food and water hot for 8 hours. Even though we are not planning to produce recipes that take over 8 hours to cook, the 24 hour flask is a good option as the heat would be more consistently retained over 8 hours. The Pioneer Vacuum Flask made by Grunwerg is perfect.

The only other piece of equipment that we reckon is essential is a preserving funnel, which avoids dangerous splashes when filling the flask with hot food. The Pioneer flask doesn’t have a wide mouth.

I suspect that if you wanted to try this recipe in a Thermos, simmering the food for a further 5 minutes before putting it into the preheated flask would probably work. I’d love to hear how it turns out if you experiment.

Leek and potato soup cooked in a vacuum flask recipe
Equipment: 1 1 litre thermos (that holds heat for 24 hours) 1 preserving funnel


1 leek (250g) washed well and sliced very fine
1 small onion (80g) peeled and sliced very fine
1 medium potato (150g) peeled and sliced fine
2 chicken stock cubes

Prepare all the ingredients and put then in the vacuum flask, pressing them down with the handle of a wooden spoon if necessary. Top up with water and pour the contents of the flask into a saucepan.
Fill the flask with boiling water, screw the top on firmly and leave for ten minutes – this will preheat the flask.
After ten minutes bring the saucepan of water and vegetables up to boiling point, empty the water from the flask and using the preserving funnel tip the contents from the saucepan into the flask.
Fit the top and the cup on the flask and leave for two hours.
Serve the soup with a little cream and hot crusty bread.

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  1. Aunt Raven

    Pioneer, which makes the best hot-for-24-hours thermos, makes a largish one with a wide mouth. I make pasta (fusli works best) in it–fill a pre-warmed thermos a bit over half full of pasta, then pour on boiling instant soup made with 2-3 packets of your favourite flavour (hot& sour and Mushroom are two favourites) and bring it up to the top with liquid.
    Screw top down, turn about a few times to mix, and lay sideways for about 30-45 minutes. The wide mouth flask cleans easily, and you can stew pieces of chicken or sausages in it. . . . There are plenty of sites giving directions for preparing various foods for thermos cooking. Always go for a 24 hour thermos over an 8 hr one — it will keep things piping hot overnight, as long as the container is full up to the top. An 8-hr container won’t.

  2. Steven Lomax

    love thermos-es!
    now this is not cookinng in one but a little idea i mulled over and decided to give it a whirl…
    on a weekend the other half and I like to get in the car and venture off into wonderful Northumberland (we live close) we always take a flask of tea and a sarni, sometimes we tempted with a sausage or bacon butty while we’re out but dont ‘cos we have our ”picnic”
    so have a go at this…..
    the wonderful Lidl do food bags for freezing and you can also boil in the bag with them, so, i though hmmmm…..
    sausage went in the over till cooked and wide neck flask filled wwith boiling water to heat, when sausage done disgard the water, stuff the food bag in and sausage inside, put hot water from the kettle around the bag, fold the bag over the sides of the flask and srew the lid down tight over the bag, its kind of a bain marie type thingy, take the buttered buns with you and three hours laer we has sausages for buns as hat as when they came out of the oven, second was a flask of tea and presto, brunch for nothing, we like frugal, recently we were in Alnmouth and looked at a little eatery two soups with roll and two teas would have been £13.70 instead we had hot sausage buns on the beach……

  3. Hi,
    It was my thermal cooker that was mentioned in the article. We are just a small family company of myself and my wife and have been selling thermal cookers for around 5 years. If I can be of any help (even if you do not want to buy one) I am always very happy to talk thermal cooking. In 2009 I published a cookbook on thermal cooking and you will find a lot of recipes on my site. One thing you will find is the length of time that you can keep the temperature about 60C is dependent on the volume of the cooker. The 4.5L we sell will drop below 60C after about 7 to 8 hours whereas the 3L version we sell lasts around 5 to 6 hours.
    If you do want to talk thermal cooking my number is 023 8084 7834. All the best Mr D

  4. Yogurt is another no brainer for a thermos flask – and a huge money saver – not to mention not needing yet another piece of kitchen equipment to clutter up the house.

  5. Fiona, thanks so much for the thermos cooking tip – our kitchen is being redone so I have no cooker except a camping gas burner at the moment and so far have done lamb stew and shin of beef in the thermos – the lamb was good and done the same day, the beef actually took two days (reheated and re – thermosed after day 1) but we ate it tonight and it was excellent. I forsee a lot of thermos cooking over the next few months, started after breakfast and eaten in the evening when the out door work is done…

  6. What a great photo and write-up. Well done Fiona!

  7. hi, Mr D’s thermal cooker and also his cake/bread making thermal cooker were reviewed recently in ‘Permaculture’ magazine, I think the cake/bread thermal cooker is in this edition and the thermal cooker in the previous, but they may be on the Permaculture website too. I think they seemed impressed by it, not cheap though.

  8. Check out this site,I use mine all the time.More expensive than a thermos but more or less indestructable.Have made bread and cakes in it too. COOKING RECIPES

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