Bring a large pot of well-salted water to a rolling boil. Cook pasta according to package directions until al dente, about 8 to 10 minutes. Before draining, scoop out 1/2 cup of the starchy pasta water and set it aside.

Pat chicken cubes completely dry with paper towels. Season generously with salt, pepper, and half the Italian seasoning.

Melt 1 tbsp butter in a large skillet over medium-high heat. Add chicken in a single layer and sear for 3 to 4 minutes per side until golden brown and cooked through to 165°F internal temperature. Cook in batches if needed to avoid crowding. Remove chicken to a plate.

Reduce heat to medium. Add remaining 2 tbsp butter to the same skillet. Once melted, add minced garlic and red pepper flakes. Stir constantly for 30 to 60 seconds until fragrant. Do not let the garlic brown.

Pour in chicken broth to deglaze the pan. Scrape up all the browned bits from the bottom with a wooden spoon.

Stir in the heavy cream and remaining Italian seasoning. Bring to a gentle simmer over medium heat and cook for 2 to 3 minutes, stirring occasionally, until the sauce thickens slightly and coats the back of a spoon.

Reduce heat to low. Add the freshly grated Parmesan gradually, a handful at a time, stirring gently between additions until fully melted and the sauce is smooth and velvety.

Return the seared chicken and any resting juices to the skillet. Add the drained pasta and toss gently with tongs until everything is evenly coated. Add reserved pasta water a splash at a time if the sauce needs loosening. Taste and adjust seasoning. Serve immediately topped with fresh parsley.