Preheat the oven 180°C / 350°F / Gas Mark 4. Grease a deep ovenproof dish, or individual dishes with a little oil.
In a frying pan cook the onions, courgettes, peppers, butternut squash, leeks (if using) and garlic in the remaining oil for a few minutes, then add the tomatoes, tomato puree, seasoning and water or stock. Bubble for 10-15 minutes.
To make the sauce, place butter, flour, milk and mustard in a small saucepan and whisk continuously over a medium heat until the sauce begins to boil and thicken. Turn down the heat to its lowest and let the sauce cook for 2 minutes. Add the grated cheese.
Into the baking dish pour one quarter of the white sauce, followed by one third of the vegetable mixture. Sprinkle in a third of the cheese and follow this with a single layer of lasagne sheets. Repeat this process, ending up with a final layer of sauce and a good sprinkling of cheese and the pumpkin seeds for added crunch.
Cook in the oven for 25-30 minutes or until golden and bubbling.