Preheat oven to 350°F (180°C). In large a skillet, fry bacon until crisp.
Reserve the bacon grease.
Once cooled, cut into bite-sized pieces.
In another large skillet, heat bacon grease over medium-high heat.
Once hot, add chicken breasts, and sauté 4 to 5 minutes each side, until lightly browned.
Place browned chicken breasts in a 9x13 10 x 10 inch (25 x 25 cm) baking dish.
Pour ranch dressing over the chicken, then top with sliced mushrooms and tomatoes, if using.
Sprinkle with cheese, green onions, and bacon pieces. Bake for 25 to 35 minutes, or until chicken is no longer pink and juices run clear.