Preheat the oven to 200°C. Line two large baking trays with baking paper.
Arrange the pumpkin, capsicum, zucchini and garlic on the prepared trays, drizzle with 2 tablespoons of olive oil and sprinkle with the salt. Roast the smaller vegetables for 40 minutes and the pumpkin for 1 hour until all the vegetables are soft. Cut the pumpkin into thick wedges. Peel and mash the garlic.
While the vegetables are roasting, heat the remaining oil in a large, deep frying pan, add the onion and cook, stirring occasionally, over medium heat for 10 minutes, until soft. Add the roasted garlic, tomatoes and vinegar and bring to the boil. Reduce the heat and cook for 25 minutes, until reduced by one-third. You want to have some moisture left in the sauce as this will help to cook the lasagne sheets. Stir in the basil leaves and olives and season with salt and pepper.
Combine the ricotta, cauliflower and milk in a bowl and season well with salt and pepper.
Spoon a little of the tomato sauce into the base of a 20 cm x 30 cm lasagne dish (this stops the lasagne sheets from sticking to the bottom of the dish). Cover with a layer of lasagne sheets, spoon over half the tomato sauce, then add half the capsicum and zucchini, top with another layer of lasagne sheets, half the spinach, half the pumpkin and half the ricotta mixture and sprinkle with half the parmesan. Continue layering, finishing with the ricotta mixture and parmesan.
Bake the lasagne for 50 minutes, until the lasagne sheets are soft and the topping is golden brown. Allow to stand for 10 minutes before slicing.