Bring a large pot of salted water to a boil and cook the pasta according to package directions until al dente. Before draining, reserve 1/2 cup of the pasta water and set aside.

Heat the olive oil in a large 12-inch skillet over medium heat. Season chicken pieces with salt and pepper, then cook for 6-7 minutes until golden brown and cooked through to 165°F. Remove from skillet and set aside.

Add the sliced leeks to the same skillet and cook for 6-8 minutes, stirring occasionally, until soft and golden brown with a sweet aroma and translucent appearance with golden edges.

Stir in the mushrooms and cook for 5-7 minutes until they release their moisture and turn tender with golden edges. Add garlic and sauté for 1 minute until fragrant.

Pour in the white cooking broth, scraping up any browned bits from the bottom of the skillet with a wooden spoon. Let simmer for 2 minutes.

Stir in the heavy cream, reduce heat to medium-low, and simmer for 3-4 minutes until slightly thickened and it coats the back of a spoon.

Gradually stir in the Gruyere cheese using a figure-eight motion, allowing it to melt completely into the sauce. Keep heat low to prevent graininess. Season with salt and pepper to taste.

Add the cooked chicken back to the skillet. Toss the cooked pasta in the skillet with the sauce, adding reserved pasta water a tablespoon at a time until you reach desired consistency. Pasta should be well-coated but not swimming in sauce.

Garnish with chopped parsley if desired and serve immediately.