In a large skillet over medium heat, heat olive oil. Add chicken and season generously with salt and pepper. Cook until no longer pink, 10 minutes per side.
While chicken is cooking, bring a large pot of salted water to a boil. Cook pasta according to package directions. Drain, reserving 1 cup pasta water, and set aside.
Transfer chicken to a plate.
Add butter to skillet and let it melt.
Next, add garlic. Cook, stirring occasionally, until soft.
Then add lemon juice, zest, and slices. Stir in crushed red pepper flakes. Pour in 1/4 cup reserved water.
Return chicken to skillet, along with spinach. Gently toss until wilted, 1 to 2 minutes.
Salt to taste.
Add pasta and toss until combined. (Add more pasta water and/or butter if desired.)
Top with Parmesan.
ENJOY!