Preheat the oven to 200°C, fan 180°C, gas 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 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 1.5 litre ovenproof casserole.
To make the dumplings, place the flour in a bowl and add the butter in small pieces. Using your fingertips, mix together until you are left with coarse crumbs. Using a fork, stir through half the cheese then slowly add cold water, a little at a time, 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.