pat dry filets, season with salt and pepper. Spray a baking dish with canola oil. Spread evenly about 1/2 cup of the marinara. Sprinkle diced peppers and onions. Layer snapper filets on top. Spread remaining marinara evenly over the filets to cover. Sprinkle to taste garlic salt and Italian seasoning. Sprinkle top with parmesan.
Bake for 20 to 25 minutes (sauce will be bubbly and parmesan will be lightly browned).