If your skillet is smaller than 14 inches, you may need to cook the green beans in two batches. To be efficient, marinate the pork while you prep the other ingredients.

Steps

1. Stir the pork and 2 tsp. each fish sauce and soy sauce in a medium bowl and refrigerate for up to 30 minutes. Meanwhile, whisk the remaining 2 Tbsp. each fish sauce and soy sauce, broth or water, lime juice, brown sugar, and cornstarch in a small bowl. Stir the garlic, black pepper, and 1 Tbsp. oil in another small bowl.
2. Heat 1 Tbsp. oil in a very large nonstick skillet over high heat until the first wisps of smoke appear. Add the pork in a single layer and cook, without stirring, until browned, about 1½ minutes; scrape it into a large bowl and set aside (pork will not be completely cooked through — it will finish cooking from carryover heat).
3. Add another 1 Tbsp. oil to the skillet, return it to high heat, and heat until shimmering. Add the beans, spread them into a single layer, and cook, without stirring, until browned, about 3 minutes. Stir and continue cooking until just tender-crisp, about 1 minute longer. Scrape into a bowl (separate from the pork).
4. Add the remaining 1 Tbsp. oil to the skillet, return it to high heat, and heat until the first wisps of smoke appear. Add the onion in a single layer and cook, without stirring, until beginning to brown, about 3 minutes. Cook, stirring, until softened, about 1 minute longer. Clear the center of skillet, add the garlic mixture and cook, mashing and stirring it, until fragrant and beginning to brown, about 1½ minutes, then stir it into the onions.
5. Whisk the sauce mixture to recombine (make sure the brown sugar is incorporated). Add the beans and sauce mixture to the skillet and cook, stirring constantly, until the sauce thickens and coats vegetables, about 40 seconds. Add the pork and cook, stirring, to heat it through, about 40 seconds. Stir in the cilantro and serve immediately with steamed rice.