Combine parsley, garlic, olive oil, red wine vinegar, oregano, salt, black pepper, and crushed red pepper flakes in a bowl. Mix well and refrigerate for at least 2 hours.

Marinate chicken in half of the chimichurri sauce for 15 minutes at room temperature.

Preheat grill to medium-high heat (around 375°F) or heat skillet with olive oil.

Grill or sauté chicken for 5-7 minutes each side until cooked through (internal temperature of 165°F).

Let the chicken rest for 5 minutes and drizzle remaining chimichurri sauce before serving.