Preheat oven to 400 degrees F (200 degrees C). Grease an 8x8-inch baking dish.
Cook the hamburger and onions in a skillet over medium heat until cooked through. Drain the grease. Stir in the garlic, ranch seasoning, bacon, and diced tomatoes. Set aside.
Cook the pasta from both boxes of Horizon(R) Macaroni & Mild Cheddar Cheese according to the package directions. Drain; add the butter and milk. Stir until creamy. Stir in the hamburger mixture. Spoon into prepared baking dish. Top with the cheese and crushed crackers.
Bake 15 minutes. Serve immediately. Top with green onions, crumbled bacon, and fresh tomatoes, if desired.