1.  In a large skillet over medium heat, heat olive oil.  Add chicken and season generously with salt and pepper.  Cook until no longer pink, about 10 minutes per side.

2. Meanwhile, bring a large pot of salted water to a boil.  Cook angel hair according to package directions until al dente.  Drain, reserving 1 cup of pasta water and set aside.

3. Transfer chicken to a plate and add butter to the skillet.  Let melt, then add garlic and red onion.  Cook, stirring occasionally, until soft and translucent, about 3-4 minutes.  Then add lemon juice, zest, and slices.  Stir in crushed red pepper flakes and cook for another 4-5 minutes or until lemon slices start to soften.  Pour in 1/4 cup reserved pasta water. 

4.  Dice the cooked chicken into 1-inch pieces and return chicken to skillet, along with spinach.  Gently Toss until wilted, 1-2 minutes. 

5. Add angel hair and toss until combined.  (Add more pasta water if desired)

6. Plate and top with grated Parmesan. 

** salmon or shrimp can be subtituted instead of chicken – also add chopped fresh herbs or cajun seasoning to change it up if desired **