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 425ยบ 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.
Notes
To prep ahead. Prepare the marinade, slice the pork, and place the pork into an airtight container in the refrigerator. Slice the peppers and onions and place them into a separate airtight container in the refrigerator. Marinate for one hour to overnight.To cook. Remove the pork slices from the marinade and cook the pork, peppers, and onions according to the recipe instructions.