Heat oil in a nonstick frying pan, add onions, and cook over medium heat, stirring frequently, until softened and starting to caramelize a bit. Add peppers and cook until they are softened. Put in the sausage, letting it sear a bit, then add liberal amounts of ketchup, salt, and pepper, stirring lots to avoid sticking (you can add a little water or turn down the heat if necessary). Keep cooking until ketchup thickens into a tasty sauce, then mound the whole mess in the middle of the pan and put a slice of cheese on top. After the cheese has melted, scoop sausage, veggies, and cheese into the bun and enjoy! :o)
Note: If you can't find vegan Italian-style sausages, or if you just want a really zippy sandwich, add some hot sauce/oregano/fennel seeds/basil/whatever Italian seasonings strike your fancy to the sauce. My carnivorous family loves this one.