Method

1. 1

   Thinly slice the onions. Melt the butter in a large oven-safe skillet over medium-low heat. Add the onions and 1/2 teaspoon salt, then cook, stirring occasionally, until deeply golden and soft, 20 to 25 minutes.

   25 minMedium-low heat
2. 2

   Meanwhile, bring a large pot of salted water to a boil. Cook the macaroni until just tender according to the package directions. Drain well.

   10 minBoiling water
3. 3

   Reduce the skillet heat to low. Stir in the milk, remaining salt, and pepper. Add the macaroni, Gruyère, and cheddar, stirring until the cheeses melt and coat the pasta, about 2 minutes.

   2 minLow heat
4. 4

   Spread the macaroni evenly in the skillet and arrange the caramelized onions over the top. Broil 4 to 6 inches from the heat until bubbling and lightly browned, 2 to 3 minutes. Watch closely to prevent scorching.

   3 minBroiler on high
5. 5

   Let the skillet rest for 5 minutes before serving.

   5 min