Preheat oven to 400°F (200°C). Spray 10-inch pie dish with cooking spray and set aside.
In a large skillet, brown sausage and onion over medium-high heat until sausage is no longer pink and onion is tender (about 8-10 minutes). Sprinkle with salt and pepper, toss, and then drain.
Place sausage mixture in prepared pie dish.
In a separate bowl, use an electric mixer (or a blender) to beat milk, baking mix, and eggs until smooth (about 1 minute with a mixer).
Pour batter over the sausage in the pie dish.
Bake for 25 minutes.
Remove from oven; top with tomato slices and sprinkle with cheese. Place pie dish back in the oven for about 5-8 more minutes, or until a toothpick inserted in the center comes out clean.
Allow to cool for 5-10 minutes before slicing and serving.