Prepare the steak. Using a sharp knife, slice the steak thinly across the grain into ¼-inch thick strips. Place in a large bowl, then season with 2 tsp kosher salt. (See note below.)
Make the sauce. Combine all sauce ingredients in a liquid measuring cup or large bowl, then whisk to combine.
Cook the veggies. Heat a large frying pan or wok over medium-high heat. When hot, add 2 Tbsp neutral oil, heat for about 30 seconds, then add the string beans and sliced onion. Cook, stirring occasionally, until they begin to blister, about 4-6 minutes. While they’re cooking, crush the peppercorns. If you don’t have a mortar and pestle, simply place them in a Ziploc bag, seal tightly, then crush with something heavy (eg, a rolling pin).
Cook the beef. To the veggies, add the beef, crushed peppercorns, and fresh cilantro, then cook, stirring occasionally, for another 4-6 minutes. You want the beef to brown a bit, but yes, it cooks quickly!
Add sauce. Pour the sauce on top, stir well, then let it bubble and reduce for a minute. Turn off the heat, add the juice of one lime, then taste and adjust seasoning as needed.
Serve. Spoon over bowls of steamed rice, then garnish with sliced scallions and sesame seeds. Enjoy!