Preheat the oven to 400°F. Grease a medium glass pie plate (9-inch) with vegetable oil.
In a large skillet, heat some oil over medium heat and fry the onion until soft, about 3 minutes.
Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned, about 10 minutes. Drain any excess fat from the beef and season with salt, black pepper, and optionally, garlic powder.
Spread the cooked and seasoned beef mixture evenly into the prepared pie plate. Sprinkle the cheddar cheese over the beef layer.
In a small bowl, whisk together the Bisquick mix (or flour), milk, and eggs until combined. Pour this batter over the beef and cheese layers in the pie plate.
Bake for 20 to 25 minutes, or until a knife inserted in the center comes out clean.
Serve warm and enjoy this comforting slice of cheeseburger pie.