This is great with the Vietnamese Herb and Salad Plate (some or all of Asian basil leaves, mint leaves, coriander sprigs, leaf letter, small scallions, lime wedges, bird chiles, cucumber, bean sprouts, carrot & daikon (fresh or pickled)).
Combine the lemongrass, garlic, shallots, and chile in a mortar. Pound into a paste. You can do this in the blender, adding a bit of water.
Transfer to a bowl and add fish sauce, lime juice, and water. Blend well. Add the sesame oil, blend, and set aside.
Cut the meat into very thin slices (less than 1/8 in if you can. This is MUCH easier partly frozen). Cut the slices into 1 1/2 inch lengths. Place the meat in the marinade. Cover and marinate 1 hour at room temperature or up to 8 hours refrigerated.
Thread the meat onto skewers and sprinkle with sesame seeds. Lightly oil the grill rack or broiler pan. Grill meat for 1 min each side (medium rare) or until thoroughly cooked.
Serve with salad and nuoc cham (Vietnamese table sauce) or nuoc leo (Vietnamese peanut sauce).