Sauté onion in oil 2-3 minutes in saucepan. Add diced tomatoes and seasonings, simmer 15 minutes. Meanwhile, cook spaghetti, drain and set aside and make crumb topping by mixing melted butter, breadcrumbs, and parmesan cheese. Add undiluted mushroom soup and olives to the tomato combination. Put alternating layers of spaghetti and sauce in greased baking dish. Sprinkle casserole with crumb topping. Bake at 350 degrees for 20-30 minutes. Serves 6