The original recipe calls for filet mignon, red onion, scallions, watercress, and a soy-based stir-fry sauce. I've used Romaine in place of the watercress and a lot more of it to turn this more into an entrée salad. The warmth of the meat and vegetables will slightly soften the Romaine, but it won't get lost.
I've made several variations of this recipe, including one where I cleaned out the fridge: I passed half a pound of Brussels sprouts and a few carrots down the shredder of my food processor; I thinly sliced some shiitake mushrooms; I sliced a jalapeño, chopped some cilantro, and finely cut up a bunch of curly kale. So, I think you can really tailor this to what you have on hand.
I also really love this salad when the beef is replaced with cubes of tofu, coated lightly in cornstarch and pan-fried until golden. They stay crisp but also soak up the flavorful sauce.
Finally, the original recipe calls for a lime dipping sauce made with fresh lime, salt and pepper. When this dish becomes more of a salad, as here, the dipping sauce doesn't work as well, but a little bit of fresh lime to taste is nice.
Place the steak in a bowl with 1 tablespoon of the oil, 1 teaspoon of the sugar, 1 teaspoon kosher salt, and pepper to taste. Toss to coat. The steak can sit like this for as long as one day in the fridge.
In a small bowl, stir together the remaining 2 teaspoons of sugar, the vinegar, mirin, soy sauce, and fish sauce. Place the Romaine in a large serving bowl.
Heat a wok or large skillet over high heat. Add 1 tablespoon of the oil, and when it shimmers, add the steak. Cook undisturbed for about a minute, then stir and cook until done, about 1 more minute. Transfer steak to a plate. Discard oil in wok, and add the remaining tablespoon of oil. Immediately add the red onion, and scallions. Cook for 1 minute, stirring occasionally. Add any other vegetables, the garlic and cook for 1 more minute or until the vegetables just begin to soften. Return the steak to the wok and add the sauce. Bring to a simmer (it should happen immediately), toss to coat, then turn off the heat.
Pour the contents of the wok over the Romaine. Let sit for a minute to allow the heat to soften the Romaine. Toss gently to combine. Serve with lime wedges if using.