Begin by slicing the yellow onions thinly and heating a large skillet over low heat. Add butter to the skillet, allowing it to melt before adding the onions. Cook the onions slowly for 30-40 minutes, stirring occasionally, until they turn a deep golden brown.

Sprinkle the flour over the caramelized onions and stir well to incorporate, cooking for about 2 minutes. Gradually whisk in the beef broth and heavy cream, ensuring a smooth mixture. Cook the sauce over medium heat until it thickens slightly, then stir in Gruyère and cheddar until fully melted.

In a large pot, bring water to a boil and add a generous pinch of salt. Add the elbow macaroni and cook until al dente, usually 7-9 minutes. Drain the pasta, reserving a cup of the starchy pasta water for later.

Once the cheese sauce is ready, gently fold the cooked macaroni into the skillet, ensuring every piece of pasta is coated. If the sauce appears too thick, add a splash of reserved pasta water.

Preheat your broiler and transfer the macaroni and cheese mixture into an oven-safe dish. Sprinkle additional Gruyère and cheddar cheese over the top and broil for 3-5 minutes until the cheese is bubbly and golden brown.

Once baked, let it cool for a couple of minutes and then garnish with freshly chopped parsley or chives before serving.