This simple tuna and vegetable casserole comes together with a simple white sauce. Frozen cooked green beans, tuna, and diced celery make a tasty casserole with relatively few carbs. If you aren't counting carbs, serve the casserole with some hot cooked rice or noodles. Canned or fresh sauteed mushrooms are another possibility for extra flavor and texture. For an all-in-one casserole, spread a layer of cooked riceabout 1 1/2 to 2 cupsin the prepared baking dish before you add the tuna and green bean mixture. Top the casserole off with some crumbled french fried onions if you'd like, or sprinkle about 1/2 cup of buttered breadcrumbs over the top before it goes into the oven.