For the tomato sauce, mince the onion and garlic and sauté in olive oil for 2-3 minutes. Add the cubed squash and sauté a little longer. Add the bouillon, tomatoes and seasonings. Simmer covered at medium temperature for about 10 minutes.
For the béchamel sauce, melt the margarine in a pot. Stir in the flour and deglaze with bouillon and oat drink (or the like). Briefly bring to a boil and cook until thickened. Season with salt, pepper and nutmeg.
Crumble the FETO by hand. Peel the red beets and squash, and cut into thin slices.
In a large baking dish (approx. 30 x 40 cm), make layers: first the tomato sauce, then a layer of pasta sheets, tomato sauce, squash, béchamel sauce, and FETO. Repeat with red beets instead of squash, and repeat again with spinach. Salt the vegetables to taste. Finish with béchamel sauce.
For the topping, crumble the FETO. Mix with the breadcrumbs, oil, Herbes de Provence and optional yeast flakes. Sprinkle over the lasagne. Convection bake at 200 °C for 35-40 minutes until the pasta has softened.