Preheat oven: Preheat your oven to 375°F (190°C).
Prepare chicken and bacon: If not done already, cook and cube or shred the chicken breasts. Cook the bacon until crispy, then chop it into bite-sized pieces.
Sauté garlic: In a large skillet, sauté the minced garlic over medium heat until fragrant, about 1 minute.
Combine ingredients: In a large mixing bowl, combine the cooked chicken, chopped bacon, sautéed garlic, ranch dressing, and half of the shredded mozzarella and cheddar cheeses. Steam or lightly blanch the broccoli florets for 3-4 minutes until tender but still crisp. Add the broccoli to the chicken mixture. Make sure the spinach is fully thawed and well-drained. Add it to the chicken mixture.
Transfer to baking dish: Spread the mixture evenly in a greased 9x13 inch casserole dish.
Top with cheese: Sprinkle the remaining mozzarella and cheddar cheeses evenly over the top.
Bake: Place the casserole in the preheated oven and bake for 20-25 minutes, or until the cheese is melted and bubbly.
Serve: Let the casserole cool for a few minutes before serving. Enjoy warm, straight from the oven!