Soak 12 wooden skewers in water. Set aside. Skip this step if using metal skewers.
Cut the meat into 11⁄2-inch chunks. Place into a large bowl. Place the Napa leaves into a shallow pan.
Place the garlic, oil, vinegar, red pepper flakes, onion powder, salt, cumin, cayenne, paprika, and lime juice into the bowl of a food processor fitted with a metal blade. Pulse to combine. This can also be done in a quart-sized container using an immersion blender. Pour half the marinade over the beef cubes and the other half over the cabbage leaves. Toss and smear to coat. Allow to stand for 10 minutes.
Crush and roll the Napa cabbage leaves to form 12 bundles. Thread a cabbage bundle onto a skewer. Add a red pepper chunk, beef cube, tomato, cabbage bundle, then a red pepper chunk, beef cube, tomato, and a scallion to cover the last 2 inches of the skewer. Repeat with remaining skewers.
Spray an outdoor grill or grill pan with nonstick cooking spray and heat over medium heat. When it is very hot, add the skewers, in batches if necessary, rotating so each side is grilled, for a total of 12–15 minutes. If the meat is still too rare, place on a cookie sheet and into a hot oven for 5 minutes.
Serve 2 skewers per plate.