Heat 1 tablespoon of the canola oil in a large skillet over medium-high heat. Season the steak with ¼ teaspoon each salt and pepper.
Cook until an instant-read thermometer inserted into the thickest steak registers 130° F, 2 to 5 minutes per side for medium-rare. Let rest for 5 minutes before slicing against the grain.
Meanwhile, whisk together the vinegar, miso, sesame oil, sugar, and the remaining 3 tablespoons of canola oil in a large bowl. Add the watercress, radishes, scallions, snap peas, and sliced steak and toss to combine.