Slice the steak. Place the leftover steak on a cutting board. Slice it very thinly against the grain.
If it’s cold, it will slice more easily. Set aside.
Prep the veggies. Thinly slice the onion and any vegetables you’re using so they cook quickly and evenly.
Heat the pan. Add the oil to a large skillet over medium-high heat. When it shimmers, add the onions and a pinch of salt.
Cook 2–3 minutes until starting to soften.
Add vegetables. Toss in the remaining vegetables. Stir-fry 3–4 minutes until crisp-tender. Don’t overcook—keep a little bite.
Stir in aromatics. Add the garlic and ginger.
Cook 30–45 seconds, stirring constantly, until fragrant.
Make the sauce in the pan. Reduce heat to medium. Add soy sauce, Worcestershire, brown sugar, vinegar, red pepper flakes (if using), and a few grinds of black pepper. Stir and let it bubble for 30 seconds.
Add the butter. Stir in the butter until melted.
This rounds out the sauce and gives it a silky finish.
Warm the steak. Add the sliced steak to the pan and toss for 30–60 seconds just until warmed through. Avoid letting it sit too long, or it may toughen.
Taste and adjust. If it needs brightness, add a splash more vinegar or a squeeze of lemon. If it needs salt, add a dash more soy sauce.
Serve. Spoon over rice or noodles, or tuck into warm tortillas for quick steak wraps.
Garnish with green onions.