In 5-quart Dutch oven, cook and drain pasta as directed on package. Return to Dutch oven; cover to keep warm.
Meanwhile, in 10-inch skillet, cook bacon over medium heat 5 to 8 minutes, stirring often, until crisp; remove to paper towels. Reserve 2 tablespoons drippings.
In 3-quart saucepan, heat butter and reserved bacon drippings over medium heat until butter is melted. Add garlic and onion; cook for 1-2 minutes or until fragrant. Stir in flour, salt and pepper with whisk until smooth. Stir in milk; heat to boiling. Stir in cheese until melted and sauce is smooth.
Pour cheese sauce over pasta; stir until coated. Stir in spinach. Reserve 1/4 cup of the bacon; stir remaining bacon into pasta mixture. Divide pasta mixture among 6 serving bowls; garnish with reserved bacon.
Garnish with cherry tomatoes, sliced green onions, or sliced baby bella mushrooms.