Eliminate the green part from the leeks, cut them in half, rinse them under running water then slice finely. Remove seeds and filaments from the pumpkin, slice it, remove the skin and cut into cubes of 1cm. Divide the cauliflower into florets.
In a saucepan over medium heat, melt the butter with the sage. Brown the vegetables for 5 minutes then add 1 cup of vegetable stock and salt then stew for about 10 minutes, adding more stock if necessary. You'll want a smooth sauce, but not too liquid. Add freshly ground pepper and salt to taste.
Bring to a light boil the butter with 300ml of the milk in a saucepan over medium heat. In a blender pour the remaining milk, 1 teaspoon salt and the flour. Blend then pour into the saucepan.
Mix well and bring to a boil. Simmer for 5 minutes, stirring often. Remove from heat, season with salt, add the nutmeg and Gruyere. Set aside.
Boil the pasta and lay to dry on the tea towels.
Grease a baking dish about 26cm long and 20cm wide. Lie down a layer of cheese sauce and cover with a layer of pasta. Spread a bit of cream sauce then with the vegetable sauce and sprinkle with Parmesan.
Continue with layers of pasta, sauces and parmesan, until all ingredients are used. Finish with a layer of cheese sauce and a sprinkling of Parmesan cheese and butter.
Bake at 180 degrees C for 35 minutes, until the lasagna is swollen and with a browned surface.