Welcome to the forum.
Often, when I have leftover ground beef with onions, I stir fry it with frozen mixed vegis and serve it over rice. If I also have cold, cooked rice, I add that while stir frying. It's a quick and tasty meal.
You can also add tomato sauce and use it for sloppy Joes or as the pasta sauce.
For the above suggestions, add seasonings that you feel would go well with the dish if you are inspired to.
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Robert A. Heinlein