Pat the steak dry and season with 1 tablespoon salt and pepper. Don’t skimp — flavor starts on day one. Let it sit 10 minutes if you’ve got time.
Heat a large skillet over medium-high and add oil. When the oil shimmers, toss in the steak in a single layer. Sear 1–2 minutes per side until browned but not cooked through. Remove to a plate.
In the same skillet, add onions and peppers. Sauté 4–5 minutes until they soften and caramelize a bit. Garlic goes in for the last 30 seconds so it doesn’t burn.
Return the steak to the pan. Stir in soy sauce, vinegar, and sugar. Cook 1–2 minutes until the sauce starts to coat the meat nicely.
Stir in the cornstarch slurry and simmer until the sauce thickens, about 1 minute. Adjust salt and pepper to taste. Garnish with parsley or scallions if you’re feeling fancy.