Place chicken in a medium bowl.
In a small bowl add olive oil, 2 teaspoons each salt and pepper and 1 clove of garlic, crushed. Pour over chicken.
Cover bowl and place in refrigerator to marinate for at least 2 hours or overnight.
Preheat grill to medium-high heat.
Remove chicken breasts from marinade.
Brush the grill grates with oil to prevent sticking.
Place chicken breasts on the preheated grill. Grill each side for approximately 6 - 8 minutes, or until the internal temperature reaches 165°F. Use a meat thermometer to ensure proper doneness without overcooking. (For attractive grill marks, rotate the chicken 90 degrees halfway through cooking on each side.)
Remove chicken from grill and keep warm.
Pour 1 tablespoon oil into a medium skillet. Place over medium heat.
When oil is hot and shimmering, add 3 cloves of crushed garlic, prepared red onions and mushrooms. Sauté until golden brown, about 2 - 3 minutes
Once brown, deglaze the pan with white wine.
Once wine has reduced, add marinara sauce. Bring to a simmer.
Add kalamata olives and chili flakes.
Taste. Add salt and pepper, to taste.
Cook until thick and bubbly.
Just before serving, add baby spinach leaves at the last minute. Toss to coat and wilt spinach. Remove from heat.
Place 1 warmed chicken breast on each serving plate.
Spoon hot sauce over chicken.
Sprinkle with Parmesan cheese.