Heat the oven to 200°C/Fan 180°C/Gas 6. Cut the sausages into thirds. Heat a large frying pan for 1 min. Add 1 tbsp oil and the chopped sausages. Fry for 4-5 mins till browned.
While the sausages brown, trim the tops and bottoms off the leeks. Halve them and rinse out any grit. Finely slice them. Peel and trim the tops off the carrots. Slice them into 1 cm rounds. Peel and slice the onion.
Add the leeks, carrots and onion to the frying pan. Cook and stir for 4-5 mins till the veg have just started to soften.
Peel and grate or crush the garlic. Drain and rinse the lentils. Put the lentils to one side.
Stir the garlic into the veg. Pour in the tin of chopped tomatoes. Half fill the can with warm water and add that too. Scatter in the thyme. Clamp on a lid and simmer for 15 mins.
Meanwhile, tear the leaves from the kale into bitesized pieces. Pop the leaves into a bowl and massage
in ½ tbsp oil and a pinch of salt and pepper.
Line a baking tray with baking paper. Scatter the kale leaves over it in a single layer. Slide into the oven and bake for 5 mins till the leaves are just crisp and a little brown. Leave to cool.
After the cassoulet has cooked for 15 mins, tip in the drained lentils. Stir and cook for 2 mins to warm through. Taste and add a little more salt and pepper if you think it needs it. Serve with the kale chips.