Lemon Butter Chicken Pasta with spinach

extra-virgin olive oil

1 lb. boneless skinless chicken breasts

kosher salt

Freshly ground black pepper

12 oz.  angel hair

1/4 c.  butter

2  cloves garlic, minced

1/2  small red onion, finely chopped

1/3 c. freshly squeezed lemon juice (from 4 lemons)

1  lemon, thinly sliced into half moons

Zest of 1 lemon

pinch of crushed red pepper flakes

3 c baby spinach