Trim and slice the beef across the grain into 1/4–3/8" (6–9 mm) strips. Pat the strips very dry with paper towels and season evenly with 1 tsp kosher salt and 1 tsp freshly ground black pepper. WHY: Patting dry removes surface moisture that would steam the meat instead of allowing a proper Maillard sear; slicing across the grain shortens the muscle fibers, making the finished bites more tender.

Warm a large heavy skillet (cast-iron or stainless steel) over medium-high heat for 2–3 minutes until hot. Add 2 tbsp olive oil and swirl to coat. WHY: A properly preheated pan and a film of oil ensure fast, even searing and prevent sticking; cast iron holds heat to sustain the sear.

Working in a single layer, add half the beef strips to the hot pan (do not crowd) and sear 1.5–2 minutes undisturbed until a deep brown crust forms. Flip and sear the other side 1–1.5 minutes for medium-rare (adjust timing for desired doneness). Transfer to a plate and tent loosely with foil. Repeat with the remaining beef. WHY: Searing in batches prevents crowding which would lower the pan temperature and produce steaming rather than browning; leaving the meat undisturbed allows a crust to develop for flavor.

Lower the heat to medium. If using onion, add it to the pan with a pinch of salt and cook 3–4 minutes until softened and beginning to brown, stirring occasionally. WHY: Sweating the onion in the residual fond picks up concentrated browned flavors and adds sweetness that balances the beef and garlic.

Add 1 tbsp minced garlic and cook 20–30 seconds, stirring constantly — just until fragrant. WHY: Garlic cooks very quickly; adding it after the onion prevents burning and keeps the flavor bright.

Pour 1 tbsp soy sauce into the pan and scrape up the browned bits (fond) with a wooden spoon to deglaze. Let the liquid reduce briefly (about 30 seconds). WHY: Deglazing dissolves the fond into the sauce, capturing concentrated savory flavors that make the sauce taste ‘meaty’ and restaurant-quality.

Reduce heat to low and add 2 tbsp butter, swirling the pan to melt and emulsify the pan sauce. Once the butter is melted, return all the beef strips to the pan and toss to coat for 30–45 seconds — just long enough to reheat and finish cooking to the desired doneness. If using, add 1 tbsp lemon juice off the heat to brighten the sauce. WHY: Finishing with cold butter (monter au beurre) gives the sauce a glossy sheen and silky mouthfeel; returning the beef briefly after the sauce is built keeps it juicy without overcooking.

Taste and adjust seasoning with salt and more pepper if needed. Plate the beef strips immediately and garnish with chopped parsley. Serve hot. WHY: Final seasoning check ensures the soy/butter balance is right; plating immediately preserves the texture and heat of the dish.