Preheat oven to 400 degrees F (200 degrees C). Grease a 9-inch pie plate.
Place a large skillet over medium heat, and cook and stir the turkey sausage, onion, green bell pepper, mushrooms, and garlic until the turkey is no longer pink, about 8 minutes. Spread the turkey mixture into the prepared pie plate.
Beat the eggs in a bowl with the milk, and stir in the biscuit mix. Pour the mixture over the turkey mixture in the pie dish. Sprinkle with Cheddar cheese.
Bake in the preheated oven until the cheese topping is browned and the pie filling is thickened and bubbling, about 30 minutes.