French Onion Mac with Caramelized Onions and Gruyere Skillet Step-by-step instructions

Before you start

Before starting, bring a large pot of salted water to a boil for the pasta. Grate the Gruyere cheese using a box grater and set it aside in a bowl. Slice the onions into thin, uniform half moons.

Have all ingredients measured and ready, as the cooking process moves quickly once the onions are caramelizing. If using a cast iron skillet, ensure it is clean and dry before heating. Preheat the oven or broiler to the required temperature so it's ready for the final baking step.

Instructions

Caramelize and Cook

1. 1Caramelize the Onions
   Melt 2 tablespoons of butter in a large cast iron skillet over medium heat. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, for 20-25 minutes until the onions are deeply golden, jammy, and caramelized. If they start to stick, add a splash of water. Transfer the onions to a bowl and set aside.
   Look for Onions are soft, deep brown, and smell sweet.
   Tip Low and slow is key; don't rush the caramelization.
2. 2Cook the Pasta
   Bring a large pot of salted water to a boil. Add the cavatappi and cook until al dente, about 8-10 minutes or according to package instructions. Reserve 1/2 cup of pasta water, then drain the pasta.
   Look for Pasta is tender but still firm to the bite.
   Tip Undercook by a minute since it will bake later.

The onions should be soft, deeply browned, and smell sweet and nutty. The pasta should be al dente, with a slight firmness in the center. The kitchen will be filled with the warm aroma of butter and onion. Before moving on, ensure the onions are not burnt and the pasta is drained well.

Make the Creamy Sauce

3. 3Make the Gruyere Cream Sauce
   In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Pour in the heavy cream and bring to a gentle simmer. Reduce the heat to low and gradually whisk in about 8 ounces of the grated Gruyere, stirring until the sauce is smooth and bubbling. Season with salt and pepper.
   Look for Sauce is thick, glossy, and coats the back of a spoon.
   Tip Add cheese off the heat to avoid a gritty texture.
4. 4Combine Pasta and Onions
   Add the drained cavatappi and the caramelized onions to the skillet with the Gruyere cream sauce. Stir gently to coat every piece of pasta with the sauce. If the sauce seems too thick, loosen it with a splash of the reserved pasta water.
   Look for Pasta is evenly coated in a creamy, onion-flecked sauce.
   Tip Fold gently so the pasta doesn't break.

The sauce should be smooth, glossy, and thick enough to coat the back of a spoon. It should have a rich, cheesy aroma and a pale golden color. The pasta and onions should be evenly coated, with no dry spots. If the sauce looks thin, it will thicken as it bakes.

Top and Bake

5. 5Top with Cheese and Breadcrumbs
   Sprinkle the remaining grated Gruyere evenly over the top of the pasta. Then scatter the crispy breadcrumbs over the cheese layer, covering the surface completely.
   Look for The top is fully covered with cheese and breadcrumbs.
   Tip Press the topping lightly so it adheres.
6. 6Bake Until Golden and Bubbly
   Place the skillet under a preheated broiler or in a 400°F (200°C) oven for 5-10 minutes, until the cheese is melted, the breadcrumbs are golden brown, and the sauce is bubbling at the edges.
   Look for Top is golden brown and the sauce bubbles around the edges.
   Tip Watch closely under the broiler to prevent burning.
7. 7Garnish and Serve
   Remove the skillet from the oven and scatter fresh thyme leaves over the top. Let it rest for 5 minutes before serving. Scoop out a generous spoonful to reveal the stretchy Gruyere cheese pull.
   Look for Cheese pulls in long, stretchy strands.
   Tip Letting it rest sets the sauce slightly for cleaner servings.

The top should be golden brown and crisp, with the cheese melted and bubbling around the edges. The breadcrumbs should be toasted and crunchy. The whole skillet will be fragrant with toasted cheese and thyme. Let it rest briefly before serving to allow the cheese to set.

Common mistakes

One common mistake is rushing the caramelization of the onions. Cooking them over too high heat will burn them before they become sweet and jammy.

Be patient and keep the heat at medium-low, stirring occasionally. Another pitfall is using pre-shredded cheese, which can result in a grainy sauce due to added starches.

Grating the cheese yourself ensures a smooth, velvety texture. Overcooking the pasta can lead to a mushy casserole; cook it just to al dente, as it will continue to cook in the oven. Finally, don't skip the rest time after baking—it allows the sauce to thicken and the flavors to meld.

How to tell it’s done

The dish is done when the top is golden brown and the sauce is bubbling around the edges. The cheese should be fully melted and the breadcrumbs toasted to a deep golden color. To check the center, insert a knife or spoon—it should come out hot and the sauce should be thick and creamy, not runny.

The pasta should be tender but still have a slight bite. Letting the skillet rest for a few minutes before serving allows the sauce to set slightly, making it easier to scoop and ensuring the cheese pull is stretchy and smooth.