Sprinkle meat with onion powder. Place in a shallow baking dish. Pour the 1/3 cup stir-fry sauce over meat. Sprinkle with pepper. Cover and chill for 1-1/2 hours.
Meanwhile, in a small bowl, whisk together mayonnaise, the 3 tablespoons stir-fry sauce, mustard, and oil. Cover and chill sauce until serving time.
For a charcoal grill, arrange hot coals around a drip pan. Test for medium-high heat above the pan. Place pork on grill rack over drip pan. Cover and grill for 30 to 35 minutes or until an instant-read thermometer inserted into the center of tenderloin registers 155 degrees F. (For a gas grill, preheat grill. Reduce heat to medium-high. Adjust heat for indirect cooking. Grill as above.)
Remove meat from grill. Cover with foil; let stand for 10 minutes before carving. (The meat's temperature will rise about 5 degrees F during standing.) Cut meat into 1/2-inch-thick slices. Serve with sauce and, if desired, pea pods and carrots. Makes 6 to 8 servings.