Preheat oven to 350 degrees.
Add olive oil to an oven-safe skillet or cast iron skillet placed over medium high heat. When the oil shimmers, add the onion and garlic and cook, stirring as needed, until softened.
Stir in ground beef, mixed vegetables, and beef gravy. Heat vegetables through.
Remove skillet from heat and top with mashed potatoes and then cheese.
Bake for 30 minutes or until the cheese is melted.