Heat the oven to 400 F (200 C) and spray a 9-inch glass pie plate with cooking spray.

Heat the oil in a skillet over medium-high heat. Add the ground beef and cook undisturbed for 4 minutes. Break up the beef and add the onions. Cook, stirring, for 4 to 6 minutes, until the beef is no longer pink and the onions are softened. Add the salt and pepper and stir. Taste and adjust the seasonings.

Whisk the Bisquick, milk, eggs, and melted butter together in a medium bowl.

Drain the beef mixture and transfer to the prepared pie plate.

Sprinkle the diced tomato and cheese over the beef and pour the egg and Bisquick mixture over all.

Bake for 25 minutes or until a knife inserted in the center comes out clean.