Trim green beans into bite-sized pieces and dice onions into a small dice.
In a large skillet, heat oil over medium heat. Brown ground beef, breaking it up as you go. When the beef is about halfway cooked (still some pink), add in diced onions and green beans. Saute for another 7-8 minutes, until the green beans and onions begin to soften.
Remove the pan from the heat and add in the sour cream, can of cream of mushroom soup, can of corn (drained), half of the shredded cheese, salt, pepper, and garlic powder. Stir to combine.
Spoon tater tot green bean casserole filling into a 9x13 oven safe baking dish. Top with the remaining cheese and arrange frozen tater tots on top.
Bake at 375 for 45 minutes, until the tater tots are crispy and the cheese underneath is bubbling.