Preheat the oven to 200°C/fan 180°C/gas mark 6. Heat the oil in a large saucepan and cook the leeks for 5 minutes until soft.
Add the potatoes and carrots. Toss with the leeks and cook for a further 1 minute. Stir in the tomatoes, lentils and tomato purée. Make up 1 litre of stock using the stock cubes, then pour in. Simmer for 15 minutes until thickened. Transfer to a 1½ litre ovenproof casserole.
Place the flour in a bowl and add the butter in small pieces. Use your fingertips to mix them together until you're left with coarse crumbs. Using a fork, stir through half the cheese. Slowly add cold water, a little at a time and mixing in as you go with the fork, until you have a smooth dough.
Divide the dough into 10 dumplings and place on top of the lentil mixture, leaving a gap between each to allow some space for them to rise and spread. Sprinkle over the remaining cheese and bake for 20-25 minutes until the dumplings are golden.