In a large skillet, melt the butter over low heat. Add the sliced onions and cook them slowly for about 30-40 minutes until they caramelize.

Sprinkle flour over the caramelized onions, stirring to coat. Gradually whisk in beef broth and heavy cream, stirring until smooth and thickened for about 5-7 minutes.

Reduce heat to low and gradually add shredded Gruyère and sharp white cheddar into the sauce, stirring until fully melted.

In a separate pot, boil salted water and cook elbow macaroni until al dente, usually 8-10 minutes. Drain and reserve some pasta water.

Combine drained macaroni with the cheese sauce, mixing well. Add reserved pasta water if sauce is too thick.

Preheat the oven to 400°F (200°C). Transfer the macaroni and cheese to a greased oven-safe dish, optionally topping with more cheese and baguette slices.

Bake for about 15-20 minutes until bubbly and golden, broiling for the last 2-3 minutes for a crispy finish.

Let cool for a few minutes, then garnish with fresh herbs like parsley or chives before serving.