Preheat the oven to 400°F and lightly grease a 9-inch glass pie plate.
Brown the ground beef and onion in a skillet, season with salt and pepper, and drain very well.
Spread the beef mixture evenly in the pie plate.
Whisk together the baking mix, milk, and eggs until smooth.
Pour the batter over the beef mixture.
Bake 25 to 30 minutes, until the center is nearly set and a knife comes out clean.
Sprinkle the cheddar over the top during the last few minutes of baking and return to the oven until melted.
Rest 5 minutes before slicing and serving.