HEAT a large skillet over medium-low heat; add the butter. Once the butter is melted, add the onion, garlic, and red pepper flakes, if using. Cook, stirring, until the onions are coated, about 30 seconds.
Add the chicken, spinach, mushrooms, tomatoes, Italian seasoning, pepper, and salt. Mix well.
Pour in the cream and stir to combine. Cover and cook until the chicken has reached an internal temperature of 165°F, about 15 minutes.