01 - Set oven temperature to 375°F. Position rack in the center.
02 - Heat vegetable oil in a large skillet over medium heat. Add chopped onion and cook for 2 to 3 minutes until softened. Stir in minced garlic and cook for 30 seconds until fragrant.
03 - Add ground beef to the skillet, breaking it up with a spatula. Cook until beef is browned and cooked through, approximately 5 to 7 minutes. Drain excess fat from the pan.
04 - Incorporate ketchup, yellow mustard, salt, and black pepper. Stir well and cook for 1 minute. Remove skillet from heat and allow mixture to cool slightly.
05 - Grease a 9-inch pie dish with cooking spray or oil. Evenly spread cooled beef mixture into the dish. Sprinkle cheddar cheese over the beef in a uniform layer.
06 - In a mixing bowl, whisk together eggs, whole milk, all-purpose flour, and baking powder until a smooth batter forms.
07 - Pour batter gently over beef and cheese, covering filling completely. Bake in the preheated oven for 25 to 30 minutes, until the top is golden and the center is just set.
08 - Allow pie to rest for 5 minutes after baking. Slice and serve while warm.