Cook the pasta and broccoli – Bring a pot of salted water to a boil. Cook macaroni according to package instructions. In the last 3 minutes of cooking, add the broccoli florets. Drain and set aside.
Make the cheese sauce – In a large pan, melt butter over medium heat. Add chopped onion and cook until soft. Stir in crushed garlic and cook for another 30 seconds.
Thicken the sauce – Sprinkle in flour and stir constantly for 1-2 minutes to cook out the raw flour taste. Slowly pour in milk while whisking until the mixture thickens.
Melt the cheese – Reduce heat to low and stir in Cheddar, Gouda, and Mozzarella until fully melted and smooth. Season with salt and pepper.
Combine everything – Add the cooked macaroni and broccoli to the cheese sauce, stirring to coat evenly. Let cook for another 2-3 minutes until everything is warmed through.
Serve hot – Enjoy as is, or add a crunchy breadcrumb topping and bake for a crispy finish!