Preheat an oven to 325 degrees F (165 degrees C). Coat the bottom of a 9×13 inch pan with cooking spray.
Combine spinach and onion in the bowl of a food processor. Process to finely chop. Add eggs, milk, and baking mix; process to mix. Pour mixture into prepared pan. Sprinkle ham and cheese over the top.
Bake in preheated oven for 35 to 45 minutes. Quiche is done when toothpick inserted in the center comes out clean. Garnish with tomato slices to serve.