Preheat oven to 350 degrees. Pour tomatoes into small saucepan, and add garlic, basil, oregano, parsley, and salt. Heat to boiling. Reduce heat, and simmer about 10 minutes.
While sauce is cooking, prepare vegetables. Lightly coat a 9 by 13-inch casserole dish with olive oil. Place zucchini rounds on bottom of dish, stacking extra rounds to make a second layer, if needed. Spread spinach leaves and onion slices on top of zucchini.
When sauce is done, pour sauce over all, making sure vegetables are coated with tomatoes and their juice. If using fresh tomatoes, chop tomatoes and place on top of spinach and onions. Mix tomato sauce, garlic, basil, oregano, parsley, and salt in a small bowl and pour sauce over all.
Bake 25-30 minutes, or until zucchini is tender. Stir well before serving with cooked brown rice, lentils, or quinoa.
Yield: 6 servings (serving size: about 1 cup)