Preheat the oven to 180°C.
First, make the béchamel sauce. Melt 100g butter in a saucepan on a medium heat. Add the 2 bay leaves and 1 whole onion cut in half. When butter is melted add 100g flour little by little while whisking, making sure it doesn't burn. When combined and smooth, start adding the litre of whle milk a little at a time while whisking. Continue to whisk and cook for a few more minutes. Add the 2g nutmeg, salt and pepper to taste, and remove from heat. Remove the onions and bay leaves before use.
Mix the 20g chopped parsley into the béchamel sauce. Blanch 400g baby spinach in boiling water and drain.
Grease an ovenproof tray (approx 25cm x 35cm) with oil. Spread a quarter of the béchamel sauce on to the base and add a layer of butternut squash. Add a third of the spinach. Top with a third of the mozzarella, ricotta and feta. Add a third of the tomatoes. Cover with a third of the lasagne sheets. Be careful they don't overlap too much as they will not cook properly. Repeat these steps twice more. Finish off with béchamel sauce and the Parmesan. Scatter the sage leaves across the top.
Bake in the oven at 180°C for approx 45 minutes or until the butternut squash is tender. If the lasagne is getting too dark cover with foil until done.