Peel and core the squash.
Save the pulp.
Place the cored squash in boiling salted water and cook about ten minutes.
Drain.
Set aside.
In a frying pan, saute the onions in hot oil for about five minutes.
Add the squash pulp.
Saute five minutes more.
Mix in the spinach and cook ten minutes longer, or until the liquid is absorbed.
Set aside.
For the white sauce, heat three tablespoons oil, add the flour and fry until flour turns a light golden color.
Gradually blend in the milk, stirring constantly until the sauce is the consistency of cream.
Add salt and pepper to taste.
Add half of the white sauce to the spinach mixture.
Season with salt, pepper and nutmeg.
Into each squash put one teaspoon each of oil and Parmesan cheese.
Then fill with the spinach mixture.
Add one teaspoon of the white sauce on top of each squash.
Dust with the rest of the Parmesan cheese.
Arrange the stuffed squash in a large pyrex baking dish.
Combine the tomato sauce, if you are using it, water, oil and salt and pour around the squash.
Bake at 350 degree about twenty to thirty minutes.
Serve in a round serving platter, garnished with fresh dill.