Whisk together the oil, vinegar, ginger, rosemary, sugar substitute, salt, pepper, and pepper flakes in a bowl. Transfer half of the marinade to a gallon-sized sealable plastic bag with the pork. Marinate the pork in the refrigerator for at least 2 hours and up to 12 hours. Refrigerate the remaining marinade.
When ready to cook, build a charcoal fire or preheat gas grill
Remove pork from the bag and discard the used marinade. Place the tenderloins on the grill and cook, turning once, until cooked medium rare or when an instant-read thermometer stuck in the thickest part reads 145 degrees F, about 10 minutes. Transfer the pork to a cutting board, cover loosely with foil, and let rest for 5 minutes. Cut each tenderloin into 8 slices on the diagonal.
Meanwhile, toss peach and tomato halves in a bowl with the unused reserved marinade. Place them cut side down on the grill, and cook until lightly browned but still firm, about 4 minutes. Transfer to the cutting board and cut each peach half into thirds.
To serve, fan 3 radicchio lettuce leaves on each of 4 plates. Place 1/4 of the romaine in the center of each. On 1 side of each plate, place 4 slices of pork and on the other side, place 3 wedges of peach and 2 tomato halves. Sprinkle with the cheese and drizzle each plate with 2 tablespoons of the reserved marinade. Garnish each plate with a rosemary sprig and serve.