In a large pot, bring salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Drain and set aside, reserving about ½ cup of pasta water.
In a large skillet, heat olive oil over medium heat. Add diced chicken and season with salt, pepper, and optional red pepper flakes. Cook for about 5-7 minutes until the chicken is browned and cooked through. Transfer to a plate and set aside.
In the same skillet, add minced garlic and sauté for about 30 seconds until fragrant. Add the chopped spinach and cook until wilted, about 2-3 minutes.
Reduce the heat to low and pour in the chicken broth and heavy cream. Stir well to combine, then gradually add the grated Parmesan cheese. Mix until the cheese is melted and the sauce is creamy.
Return the cooked chicken to the skillet and toss to coat in the sauce. If the sauce is too thick, add a little reserved pasta water to reach your desired consistency. Add the cooked pasta and toss everything together until evenly coated.
Taste and adjust seasoning with salt and pepper if needed.
Notes
Serve hot, garnished with fresh basil leaves and extra Parmesan cheese.