Place the whole peeled tomatoes in a blender or food processor and give it a quick pulse just enough to puree them.
In a medium saucepan, heat some vegetable oil to medium high and sauté the onion. (Leave the onion in a large chunk so it can be removed later.)
Pour in the blended whole peeled tomatoes and simmer with the onion for about 10 minutes and set aside.
In a medium pot on medium-low heat melt the butter and whisk in the flour until smooth.
Meanwhile, warm up the two cups of milk in a separate pot or in the microwave till close to a boil.
Slowly add the warm milk while stirring.
Stir the mixture continuously between 5-10 minutes until the mixture becomes creamy in texture.
Stir in the salt and nutmeg and set aside.
Defrost the chopped spinach and drain most of the water.
In a large bowl using a hand blender or in the blender on low speed, mix the ricotta, chopped spinach, salt, nutmeg and pepper.
Boil the manicotti or canelloni pasta for about 5-7 minutes until done. Strain and let cool enough to handle.
With a spoon fill the tubes with the spinach-ricotta filing and place in a pyrex or aluminum baking pan.
Ladle over top both the tomato sauce and bechamel and top with parmesan cheese.
Broil on high for about 5 minutes to melt the parmesan cheese.