Cook the canelones pasta (or similar pasta) according to the package.
Heat the 1/4 cup oil in a large saute pan over medium heat. Cook the mushrooms for about 3 to 4 minutes. Transfer them to a clean plate.
Using the same pan, add another 1/4 cup oil and cook the onion and the pepper. Cook for 12 to 15 minutes.
Add 1 teaspoon of salt and 1/2 teaspoon of pepper.
Add the spinach and the garlic to the onion mixture. Cook for about 5-7 minutes or until the spinach is almost cooked. Add 1/2 teaspoon of salt.
Add the mushrooms and the canned tomatoes. Stir and simmer for 7 to 10 minutes.
Add this mixture to the food processor or blend it up with a hand blender and blend it briefly, leaving some chunks (you might need to do this step in batches).
Allow the mixture to cool a little bit.
Stuff the mixture in the middle of the canelones dough, once done, roll it up and transfer to a baking sheet.
At this point, you can freeze the Canelones for up to 3 months.
Pour a little bit of the sauce on the bottom of the baking dish and then add the canelones in a line (or two lines depending on the size of your baking sheet.)
Add only ONE layer of canelones.
The canelones can be touching one another,but make sure they are not overlapping. Add some of the white sauce, the grated cheese, and small pats of butter on top (butter is optional).
Bake for about 10 minutes at 350 F, and then broil it for 2 minutes to brown the cheese.