Combine 1 tablespoon olive oil, minced garlic, and chopped parsley in a small bowl. Rub over the pork sirloin, cover with plastic wrap and store in the refrigerator to marinate for at least one hour to overnight.
Preheat grill to 425º F. Arrange pork sirloin slices, peppers and onions on the grill or in a grill pan brushed with olive oil to prevent sticking. Grill pork loin about 5 minutes on each side and then flip to cook about 5 minutes on the other side. Remove from grill and allow to rest about 3 to 5 minutes before serving.