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

Sprinkle 2 tablespoons of all-purpose flour over the caramelized onions, stirring to coat, and cook for 1–2 minutes. Gradually whisk in 2 cups of broth and 1 cup of heavy cream until the mixture is smooth and thickened. Stir in 1 cup each of Gruyère and sharp white cheddar cheese until creamy and velvety.

Boil salted water in a large pot, add 8 ounces of elbow macaroni and cook until al dente, about 7–9 minutes. Drain the pasta, reserving about ½ cup of starchy water.

Combine the drained macaroni with the cheese sauce in the skillet, stirring gently until well-coated. Use reserved pasta water to adjust sauce consistency if needed.

Preheat the broiler to high. Transfer the mixture to an oven-safe baking dish, top with more cheese and toasted baguette slices. Broil for about 3–5 minutes until bubbling and golden brown.

Remove from the oven, cool for a few moments, and garnish with fresh parsley or chives before serving.