Easy and Delicious Fettuccine with Chicken and Spinach

1/2 c butter
1 onion, sliced
2 tbs chopped fresh basil, or 1½ tsp dried basil, crumbled
2 garlic cloves, minced salt and pepper
1/2 tsp dried red pepper flakes
1 lb boneless skinless chicken, sliced
2 10oz pkgs frozen spinach, thawed and squeezed dry
1 lb fettuccine noodles

Melt butter in skillet over medium heat. Add onion, basil, garlic and red pepper flakes and cook until onion is soft, about 7 minutes. Add chicken and sauté until cooked through, about 10 minutes. Mix in spinach. Season with salt and pepper. Serve over cooked fettuccine.