Begin by bringing a large pot of salted water to a boil. Add the penne pasta and cook according to the package instructions until al dente. Once cooked, reserve about 1 cup of the pasta water, then drain the pasta in a colander and set it aside.
In a large skillet, heat the olive oil over medium heat. Once hot, add the diced chicken breast. Season with salt, black pepper, and red pepper flakes, if using. Sauté the chicken for about 5-7 minutes or until it’s cooked through and no longer pink in the center.
Next, add the minced garlic to the skillet with the chicken. Sauté for about 1 minute, until fragrant. Then, add the fresh spinach. Stir and cook until the spinach is wilted, which should take about 2-3 minutes.
Pour in the heavy cream, stirring to combine all the ingredients in the skillet. Let the mixture simmer for 2-3 minutes. Then, gradually add the grated Parmesan cheese, stirring continuously until the cheese is melted and the sauce is smooth.
Add the drained penne pasta to the skillet with the creamy chicken and spinach mixture. Toss everything together until the pasta is well coated in the sauce. If the sauce is too thick, add reserved pasta water a little at a time until you reach your desired consistency.
Once everything is combined, taste and adjust the seasoning if necessary. Serve your Chicken and Spinach Penne Pasta warm, garnished with extra Parmesan cheese and a sprinkle of red pepper flakes for added heat if desired.