Pound chicken breasts or thighs to an even 3/4-inch thickness using a meat mallet or rolling pin.
If using large chicken breasts, cut them into smaller, uniform 5-6 ounce pieces to cook evenly.
In a small bowl, combine 2 tablespoons of chopped herbs, 1 tablespoon of garlic, 1 1/4 teaspoons of kosher salt, 1/2 teaspoon of cracked pepper, 1/4 teaspoon of chili flakes (optional), and 1/4 cup of olive oil. Mix well.
Coat the chicken in the marinade. Cover and refrigerate for at least 1 hour, preferably 2-3 hours.
Preheat grill to medium-high heat (375°F). Oil the grates to prevent sticking.
Grill chicken 7-8 minutes per side, or until internal temperature reaches 165°F and a light char forms. Baste with remaining marinade if needed.
Let chicken rest 5-10 minutes before serving. Garnish with 2 tablespoons of additional olive oil and lemon slices if desired.