1 pound ground beef
1½ cups chopped onion
1½ cups milk
¾ cup baking mix (such as Bisquick ®)
½ teaspoon Italian seasoning
½ teaspoon salt
½ teaspoon ground black pepper
2 tomatoes, sliced
1 cup shredded Cheddar cheese
Preheat oven to 400 degrees F (200 degrees C). Grease a deep-dish 9-inch pie dish.
Heat a large skillet over medium-high heat. Cook and stir beef and onion in hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Spread ground beef mixture into the prepared pie dish.
Blend milk, baking mix, eggs, Italian seasoning, salt, and pepper together in a blender until smooth, about 15 seconds; pour over beef mixture.
Bake in the preheated oven until biscuit topping is lightly browned, about 25 minutes. Arrange tomato slices on biscuit topping; sprinkle with Cheddar cheese. Continue baking until cheese is melted, 5 to 8 more minutes. Cool 5 minutes before slicing.