Heat olive oil in a large deep skillet or Dutch oven over medium-high heat.
Add the chicken pieces to the skillet, season with salt and black pepper, and cook for 5-6 minutes until browned and cooked through. Remove chicken from the skillet and set aside.
Reduce heat to medium. Add minced garlic to the same skillet and sauté for 30 seconds until fragrant.
Pour in the chicken broth and whole milk, stirring to combine and scraping up any browned bits from the bottom of the skillet.
Add the broken fettuccine pasta, dried Italian seasoning, and crushed red pepper flakes to the skillet. Stir to submerge the pasta in the liquid.
Bring the mixture to a gentle boil, then reduce heat to medium-low and cover the skillet with a lid.
Simmer for 12-15 minutes, stirring occasionally, until the pasta is tender and most of the liquid has been absorbed.
Return the cooked chicken to the skillet and stir in the grated Parmesan cheese until melted and the sauce is creamy.
Add the chopped baby spinach and cook for an additional 1-2 minutes until the spinach is wilted.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve hot, garnished with extra Parmesan cheese if desired.