Why a flask of soup changes a winter walk

There is a particular kind of misery that comes from sitting on a cold bench in February, eating a squashed sandwich with numb fingers. A flask of hot soup cures it entirely. It weighs little, it costs pennies to make, and it turns a mid-walk break from something you endure into something you look forward to. On a grey day on the heath, with the wind coming across open ground, a mug of something thick and warming is worth more than any energy bar.

The trick is to make soup that still behaves itself two or three hours later, in a steel tube, at the bottom of a rucksack. That means choosing the right vegetables, blending it smooth enough to pour, and getting a few practical details right before you leave the house.

Preheat the flask or lose half your heat

This is the step most people skip, and it is the one that matters most. A cold stainless steel flask will pull heat out of your soup immediately, and you will be drinking lukewarm broth by the time you stop. Fill the flask with boiling water, screw the lid on, and leave it standing for five minutes while your soup finishes. Tip the water out, fill straight away with hot soup, and screw the lid down firmly.

• Fill to within a centimetre of the rim. Trapped air is what cools soup down.
• Use a wide-necked flask if you can — easier to fill, easier to clean, and easier to drink from.
• Wrap the flask in a spare fleece or tea towel inside your rucksack for extra insulation.
• Do not open it for a look. Every time you unscrew the lid, you let the heat out.

Choose sturdy vegetables and lentils

Soup for a flask needs body. Thin, watery soups go cold quickly and slosh about unhelpfully. You want roots and pulses that hold their shape in the pan and thicken the liquid as they cook.

Good choices: carrots, parsnips, swede, celeriac, butternut squash, celeriac, leeks, and onions. Red lentils are excellent because they collapse into a natural purée and give the soup a velvety texture without any cream. Green and brown lentils stay firm and add a pleasant bite. Split peas, pearl barley and a handful of oats all thicken things nicely.

Choices to avoid: pasta and rice, which swell in the flask and drink up all your stock; delicate greens like spinach, which turn bitter and slimy; and cream or milk, which can split when held at high temperatures for hours. If you want a creamy finish, stir a spoonful of coconut milk or a knob of butter into the pan at the end, or leave the dairy out altogether and rely on blended pulses instead.

Blend it smooth for easy pouring

A chunky soup is lovely in a bowl at home and a nuisance in a flask. Lumps block the neck, and anything wider than a pea will not pour cleanly. A stick blender is the simplest tool — pulse in short bursts, adding hot stock or water a little at a time until the soup falls off the end of a spoon in a slow ribbon. Aim for the consistency of single cream: thick enough to feel substantial, thin enough to pour without shaking.

If you want it really silky, push it through a sieve, though you will lose a bit of fibre and a fair amount of time. Either way, taste and season generously before it goes in the flask. Flavours flatten when you are cold, so be bolder with salt, pepper, and warm spices than you would be at your own table. A pinch of cumin, a grating of ginger, or a teaspoon of smoked paprika all survive the journey well.

Packing it, and drinking it outdoors

Let the soup cool for two or three minutes off the heat, then fill the preheated flask and seal it. A small cup that fits over the flask is worth carrying; drinking directly from a hot flask neck is a good way to burn your lip. A spoon is useful if your soup is on the thick side.

When you stop, find somewhere sensible. Heathland is fragile — its thin, sandy, acidic soils support rare plants, nightjars, Dartford warblers and silver-studded blue butterflies, and they recover slowly from damage. Sit on a mat or a stone rather than on the heather itself, and keep to the paths where the ground is already worn. Open fires and stoves are completely out of the question on heathland at any time of year, and especially in a dry winter, when dead bracken and gorse can go up in seconds.

Take every scrap home with you. Soup dregs poured onto the ground attract scavengers and add nutrients that do not belong there. The flask, the cup, the spoon and the crusts all go back in the rucksack.

Three combinations that always work

• Carrot, red lentil and ginger. One onion, four carrots, a mug of red lentils, a thumb of grated ginger, a teaspoon of ground cumin, and a litre of stock. Simmer twenty-five minutes, blend smooth.
• Swede, parsnip and green lentil. Roast the roots first for depth of flavour, then simmer with lentils, thyme and plenty of black pepper.
• Butternut squash and split pea. Sweet, thick and filling, with smoked paprika and a little chilli to warm you from the inside on the coldest days.

Make a big pan on a Sunday, fill the flask on a Tuesday, and walk out onto the heath knowing that the best part of the day is waiting in your rucksack.