Make the pickled onions. Whisk together the rice vinegar with boiling water, sugar, and salt until the sugar has dissolved. Add the red onions and let sit for at least 30 minutes or overnight.
Season the steak with salt, pepper, garlic, and ginger. Toss in cornstarch, shaking off the excess. Let sit for 10 minutes.
Meanwhile, make the vinaigrette. Whisk together all of the ingredients for the vinaigrette (sesame - black pepper) and set aside.
Heat 1 tablespoon oil in a large saute pan over high heat. Cook the steak in the pan until the desired degree of doneness, making sure to brown all sides. Remove from heat and transfer the beef to a plate.
Top the steak with tomato wedges, pickled onions, and fresh herbs. Pour the vinaigrette on top and serve immediately.