Place all ingredients except chicken in a a food processor and blend until leaves are chopped into about 1/8" pieces (or desired size).
Pour 1/2 of the chimichurri over the chicken in an airtight container or resealable bag. Marinate for at least 1 hr or overnight.
Brush hot grill pan or grill grates with olive oil. Grill marinated chicken for about 5-8 minutes per side (depending on thickness) or until the chicken is cooked through.
Serve chicken with remaining chimichurri.