Cook and brown ground beef and onion, until fully cooked and onion is tender. Drain off any excess fat. Add taco seasoning mix and stir in salsa.
Place beef mixture into the bottom of a 9 x 13 casserole dish. Spread it out evenly with a spoon. Cover with an even layer of frozen Tater Tots® potatoes. Top with cheese, black olives, and green onion.
Bake at 350 degrees, uncovered for 15-20 minutes, or until cheese is melted and casserole is nicely browned on top. Serve with additional salsa and some sour cream.
Note: If you prefer crispier Tater Tots, bake them according to the package directions before adding them to the casserole. I think this simple extra step is worth the effort.