Place a large skillet over heat. Crumble Italian sausage into pan, stir to start to brown. Lower heat to render out fat and let meat brown.
Dice onion, add to pan. Stir to coat with fat, then saute until onion softens and starts to turn color.
Cut zucchini and eggplant into bite sized pieces. Add to pan and stir. Add can of tomatoes.
Optional:add 1/4 c water, if you want a sauce.
Simmer 20-30 minutes until vegetables are soft and flavor melds.
Serve at once. If you choose, spoon any liquid into a sauceboat to pass separately.