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Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or cooking spray.
Cook the elbow macaroni or pasta according to the package instructions until al dente. Drain and set aside.
In a large skillet, melt the unsalted butter over medium heat. Add the thinly sliced onions and cook, stirring occasionally, for about 20-25 minutes, or until the onions are caramelized and golden brown.
Add the minced garlic to the skillet and cook for an additional 1-2 minutes until fragrant.
Sprinkle the flour over the caramelized onions and garlic. Stir to coat the onions evenly.
Gradually pour in the beef broth, stirring constantly, until the mixture thickens and comes to a simmer.
Reduce the heat to low and simmer for 5-7 minutes, stirring occasionally, until the sauce is slightly thickened.
Remove the skillet from heat. Stir in the shredded Gruyere, Swiss, and mozzarella cheeses until melted and smooth. Season with salt and black pepper to taste.
Add the cooked macaroni or pasta to the skillet with the cheese sauce. Stir until well coated.
Transfer the mac and cheese mixture to the prepared baking dish, spreading it out evenly.
Sprinkle grated Parmesan cheese over the top of the mac and cheese.
Bake in the preheated oven for 20-25 minutes, or until the top is golden brown and bubbly.
Once done, remove the French Onion Mac and Cheese from the oven and let it cool for a few minutes before serving.
Garnish with chopped fresh parsley, if desired, before serving
