In a large skillet, heat oil. Once hot, add onion and cook until translucent, 5-7 minutes. Add ground beef to skillet with onion. Cook, breaking up with a wooden spoon, until browned, another 5-7 minutes. Stir in gravy and adjust seasoning.
Place frozen corn and green beans in an even layer in the bottom of a cast iron skillet. Pour the sausage gravy mixture on top of the vegetables and spread out to even. Top with tater tots in a circular shape, covering the whole surface of the skillet.
Bake until bubbly and the tater tots are golden, about 45-55 minutes.