Trim excess fat from beef tenderloin and cut into 1-inch cubes.

In a bowl, combine beef with chopped garlic, half the sugar, 1 teaspoon salt, 1/4 teaspoon pepper, and 1 tablespoon oil.

Marinate for about 2 hours (refrigerate if kitchen is warm).

In a separate bowl, combine rice-wine vinegar, remaining sugar, wine, soy sauce, and fish sauce.

Taste and adjust seasoning with salt and pepper if needed.

Mix about 1 tablespoon salt and 1 teaspoon pepper in a small bowl.

Divide the marinated beef, sliced red onion, and scallions into 2 portions.

Heat a wok or large skillet over maximum heat.

Add about 2 tablespoons oil to the hot wok.

When the oil smokes, add one portion of the beef in a single layer.

Let the beef sit until a brown crust forms, then turn to brown the other side (should take less than 5 minutes).

Add half the onion and half the scallions to the wok, and cook, stirring, for about 30 seconds.

Add half the vinegar mixture, and shake pan to release the beef, stirring if necessary.

Add half the butter, and shake pan until butter melts.

Remove the cooked beef from the wok and set aside.

Repeat the cooking process with the remaining beef, onion, scallions, vinegar mixture, and butter.

Serve the beef over watercress or lettuce leaves.

Pass the salt and pepper mixture and lime wedges at the table for seasoning.