Pat the beef strips very dry with paper towels and season evenly with 1 1/2 tsp kosher salt, 3/4 tsp black pepper, smoked paprika, ground cumin and cayenne (if using). Drying the meat removes surface moisture so the strips can sear instead of steam; an even spice coating ensures every bite has balanced flavor.

Heat a large heavy skillet (cast-iron preferred) over medium-high heat until hot — about 2–3 minutes. Add 1 tablespoon of olive oil and swirl to coat. A very hot pan is essential to create Maillard browning (the deep brown crust that gives rich flavor).

Working in a single layer (do not overcrowd), add half the beef strips in a single layer and let them sear untouched for 1 to 1 1/2 minutes, then flip and sear 45–60 seconds more for medium-rare to medium depending on thickness. Remove to a resting plate and tent loosely with foil. Quick, high-heat searing develops a caramelized crust that locks in juices; removing to rest prevents carryover cooking and keeps strips tender.

Add the remaining 1 tablespoon olive oil and sear the second batch of strips the same way. If any fond (brown bits) develops on the pan, that will become the flavor base for the sauce — leave it there.

Turn the heat down to medium. Add the butter and sliced onion to the pan and cook, stirring occasionally, until the onion is deep golden and caramelized, about 6–8 minutes. Add the minced garlic during the last 60 seconds. Browning the onion in the same pan collects those meaty browned bits (fond) and adds sweetness to balance the spices.

Pour in the beef broth and Worcestershire sauce, scraping the browned bits from the bottom with a wooden spoon (deglazing). Bring to a simmer and reduce 1–2 minutes. Deglazing dissolves the fond into the liquid, building a glossy, savory pan sauce.

If you prefer a slightly thicker sauce, whisk the cornstarch with 1 tablespoon cold water then stir the slurry into the simmering sauce and cook 30–45 seconds until slightly thickened. Cornstarch adds body quickly without changing flavor; use sparingly to avoid a gluey feel.

Return all the beef strips and accumulated juices to the pan, stir to coat in the sauce and warm through for 30–60 seconds — just enough to reheat, not overcook. Finish with lemon juice and the remaining 1/2 tsp kosher salt and a short grind of black pepper. Bright acid at the end lifts the whole dish and balances fat.

Transfer to a warmed platter, sprinkle with chopped parsley, taste and adjust seasoning, and serve immediately. Rested, sauced beef strips will be juicy with a caramelized edge and a layered savory-spicy finish.