Preheat the oven to 350°F and lightly grease an 8-inch pie dish with cooking spray.
In a large skillet, cook the ground beef over medium heat until it is fully cooked through. Then add in the onion, garlic, mushrooms, bell peppers, and zucchini. Cook for 5 minutes or until the vegetables are softened.
Remove the skillet from the heat and transfer the beef and vegetable mixture into the prepared pie dish. Sprinkle with cheese and spread it evenly across the top.
In a medium bowl, whisk together the eggs, milk, salt, and pepper. Pour this egg mixture over the beef and vegetables in the pie dish.
Bake the pie for 25-30 minutes, or until the center is set and the cheese is melted and bubbly. Let cool slightly before serving.