Preheat the oven to 400 degrees F (200 degrees C). Spray 9-inch glass pie pan with cooking spray.

Cook beef and onion in a nonstick skillet over medium heat, stirring occasionally, until beef is browned and crumbly, 8 to 10 minutes.

Meanwhile, whisk milk, Bisquick, and eggs together in a small bowl.

Drain beef and onion mixture; transfer to the prepared pie pan. Stir in salt, sprinkle with Cheddar, and pour Bisquick mixture over top.

Bake in the preheated oven until a knife inserted in the center comes out clean, about 25 minutes.