Heat oil in skillet, and saute onions for 5 minutes.
Add tomato puree, salt, pepper, oregano, and basil.
Cover and simmer over low heat for 40 minutes.
Add a little water if sauce becomes too thick.
Sift flour and salt together in a mixing bowl.
Make a well in the center and pour in spinach puree and eggs.
Mix well to form a stiff dough; if dough is too dry, add a little water.
Roll out dough 1/4 inch thick on a floured board.
Cut into 4 inch squares.
Arrange squares on a board, or pan, and dry until brittle.
Gently boil a few squares of dough at a time in salted water until tender, 6 to 8 minutes.
Drain well.
In a buttered baking dish, arrange successive layers of lasagna squares, tomato sauce, Mozzarella, and Parmesan cheese.
Make at least three layers, using the ingredients in the same order.
Bake lasagna in a 350 degree oven for 25 minutes.