Preheat oven to 375 degrees. Grease a pie dish or baking dish.
In a skillet over medium heat, cook ground beef and onion until beef is browned and onion is soft. Drain excess fat. Stir in crumbled bacon.
Spread beef mixture evenly in the prepared dish. Sprinkle cheddar and mozzarella cheese on top.
In a bowl, whisk milk, eggs, flour, salt, pepper and garlic powder until smooth. Pour evenly over the meat and cheese.
Bake uncovered for 30 - 35 minutes, until set in the center and golden on top.
Let rest 5 minutes before slicing. Serve warm with your favorite burger toppings.