Preheat the oven 180°C/350°F/gas 4. Grease a deep ovenproof dish, or individual dishes with a little oil.
In a frying pan add the onions, courgettes, peppers, butternut squash, leeks and garlic in the remaining oil for a few mins, then add the herbs, tomatoes, tomato puree, seasoning and water or stock. Bubble for 10-15 mins.
To make the sauce, place butter, flour, milk and mustard in a small saucepan and whisk continuously over medium heat until the sauce begins to boil and thicken. Turn down the heat to its lowest and let the sauce cook for 2 mins. Add the grated cheese, 1/4 of the white sauce, and 1/3 third of the vegetable mixture. Sprinkle in 1/3 of the remaining 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 mins or until golden and bubbling.