In a large skillet, melt 4 tablespoons of butter over low heat. Add 3 sliced yellow onions and cook for 30-40 minutes, stirring occasionally until they are golden brown and beautifully caramelized.

Once the onions are caramelized, sprinkle 1/4 cup of all-purpose flour over them, stirring well to combine. Gradually whisk in 4 cups of beef broth and 1 cup of heavy cream. Cook this mixture over medium heat until it’s smooth and thickened, about 5-7 minutes.

Stir in 1 cup of grated Gruyère cheese and 1 cup of sharp white cheddar cheese, mixing until melted and creamy.

While the sauce is thickening, bring a large pot of salted water to a boil. Add 12 ounces of elbow macaroni and cook until al dente, usually around 7-9 minutes. Be sure to reserve about 1 cup of pasta water before draining.

Return the drained macaroni to the pot, then pour the cheese sauce over the pasta. Gently stir to combine, ensuring every piece of elbow macaroni is evenly coated. If needed, gradually add reserved pasta water.

Preheat the oven's broiler. Transfer the mac and cheese mixture into an oven-safe dish. Top with extra cheese and broil for about 3-5 minutes until the top is bubbly and golden brown.

Remove from the oven, let it rest for a couple of minutes, and sprinkle fresh parsley or chives on top. Serve hot.