Meanwhile, cook the lasagne slices in boiling water for 3 minutes and drain. Put a sheet of lasagne in a small baking dish, add a layer of butternut squash and half the spinach. Season. Add another layer of lasagne and the rest of the butternut squash and spinach. Season and follow with a final layer of lasagne. Rip the mozzarella into pieces and scatter it over the final sheet, grill briefly until the mozzarella bubbles and browns.