POUND GARLIC, PEPPERCORN, coriander root and salt into a paste; add oil and mix together. Rub the garlic mixture over the beef; marinate for 30 minutes. Broil or grill beef until medium rare. Slice into 2-by-1/4-inch strips. Set aside in a large mixing bowl. Quickly char tomatoes under a hot broiler, turning occasionally. Do not overcook. Cool. Cut into wedges and add to the beef. Evenly spread the sliced onions on 1/2 of the sheet of foil. Fold in half, seal the edges to form a flat parcel. Place directly on top of a medium-high stove burner for 1 minute--it should make sizzling sounds. Turn over; cook for about 30 seconds longer until charred. Remove, unwrap and cool. Add to the beef mixture. Line a platter with the large lettuce leaves. Shred remaining leaves and scatter them on top. TO PREPARE THE DRESSING: In a hot, ungreased skillet, toast optional ground dried shrimp until fragrant, about 10 seconds. Reduce to medium heat. Add garlic, chiles, roasted chile sauce (nam prik pao), fish sauce, lime juice and sugar; stir together until dissolved. Cool. Add beef mixture, mint and coriander to wok; toss together gently. Pour mixture over lettuce. Serve at room temperature.