Bring a large pot of salted water to a rolling boil. Add 12 oz dried pasta and cook until al dente according to package directions (usually 9–11 minutes). Reserve 1 cup of the pasta cooking water, then drain the pasta and set aside.

Prepare chicken

While pasta cooks, pat 1 lb chicken breasts dry and season both sides with 1 tsp salt and 1 tsp black pepper.

Cook chicken

Heat 1 tbsp olive oil in a large skillet over medium-high heat. When shimmering, add the seasoned chicken breasts and cook 5–7 minutes per side until golden and internal temperature reaches 165°F (74°C). Remove chicken to a cutting board and let rest 5 minutes, then slice or dice into bite-sized pieces.

Cheese Sauce

Make roux

In the same skillet (wipe out excess oil if there’s too much, leave any fond), reduce heat to medium and add 3 tbsp unsalted butter. When melted, add 3 tbsp all-purpose flour and whisk constantly for 1–2 minutes until bubbly and slightly nutty (this cooks out the raw flour flavor).

Add liquids

Gradually whisk in 2 cups whole milk and 1 cup chicken broth, adding a little at a time to prevent lumps. Continue whisking until the mixture is smooth.

Simmer and flavor

Add 2 minced garlic cloves, 1 tsp Dijon mustard, 1 tsp Italian seasoning, 1 tsp salt, and 1/2 tsp black pepper. Bring sauce to a gentle simmer and cook 3–4 minutes, whisking, until it thickens to a silky cream sauce that coats the back of a spoon. If sauce becomes too thick, whisk in up to 1/2 cup of the reserved pasta water to reach desired consistency.

Add cheeses

Reduce heat to low. Gradually stir in 1.5 cups grated sharp cheddar, 1 cup grated mozzarella, and 1/2 cup grated Parmesan, adding a handful at a time and stirring until melted and smooth. Taste and adjust seasoning, adding more salt or pepper if needed.

Finish & Serve

Combine pasta, chicken, and sauce: Add the drained pasta and sliced/diced chicken to the skillet with the cheese sauce. Toss gently to coat everything evenly. If the sauce is too thick, add reserved pasta water 1–2 tbsp at a time until you reach a glossy, saucy consistency.

Garnish and serve: Sprinkle with 2 tbsp chopped fresh parsley and 1/4 tsp red pepper flakes if using. Serve immediately, 4 generous servings.