Lemon Butter Chicken Pasta

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. 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 pasta water, and set aside. Transfer chicken to a plate and add butter to skillet. Let melt, then add garlic and red onion. 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. Add angel hair and toss until combined. (Add more pasta water if desired.) Top with Parmesan.