Add the beef slices into a bowl. Whisk together the vegetable oil, soy sauce, garlic, sugar, and salt and pepper to taste. Pour over the beef and marinate at room temperature for 30 minutes or longer (2 hours is good).
In a medium bowl, whisk together the rice vinegar, mirin, soy sauce, dark soy sauce, fish sauce, and sugar. Set aside.
Heat the 2 tablespoons of vegetable oil in a wok or large pan to high heat until simmering (do not let it smoke). Add the beef in batches in a single layer and sear each side, about 1-2 minutes per side, until the beef has a nice brown crust and is done to your liking. Remove the beef and set aside onto a plate.
Add the red onion and scallions (with a bit more oil if needed) and stir fry for 1 minute.
Add the garlic and stir fry for 30 seconds.
Add the sauce mixture along with the reserved seared beef. Shake the pan as you cook for a few seconds to warm through.
Serve over a mix of watercress or other salad greens, cucumbers, tomatoes.
For the dipping sauce, whisk together the lime juice, salt, and pepper, and serve alongside the beef salad.