Preheat the oven to 180°C and grease a baking dish with butter.
Boil the lentils with a crumbled stock cube for 20 minutes until tender, then drain.
Fry the onion, garlic, mushrooms, and leek in oil and butter until softened.
Add lentils and tomatoes to the vegetables then simmer for 15 minutes.
Melt butter in a saucepan, whisk in flour, milk, and cream until smooth.
Stir cheddar into the sauce once thickened and season with salt and pepper.
Layer lentil ragù, sauce, and pasta sheets in the dish, finishing with cheese.
Bake for 40 minutes at 180°C until golden-brown and serve with green salad.