In a mixing bowl, combine oil, Sriracha, fresh ground pepper, and soy sauce, stirring until well blended. Add the steak strips, ensuring they are thoroughly coated in the marinade. Cover the bowl and let it sit at room temperature for at least 30 minutes.

Heat a cast iron skillet over medium-high heat until hot. Add butter and let it melt, foaming slightly. Season the marinated steak strips with salt and pepper before placing them in the skillet. Sear for 1–2 minutes on each side until a golden crust forms.

Remove the steak from the skillet and set aside. Lower heat to medium, add another tablespoon of butter to the same skillet. Once melted, add chopped shallots and minced garlic, sauté for about 2 minutes until shallots are translucent and garlic is aromatic.

Stir in thyme, chives, and chili pepper flakes to the skillet, cooking for another minute. Add lemon juice, Dijon mustard, and beef stock, scraping up browned bits from the skillet. Allow sauce to simmer for about 5 minutes until reduced in half.

Return the seared steak to the skillet, coating each piece in sauce and heating for an additional 1–2 minutes until desired doneness. Taste sauce and adjust seasoning if needed.

Plate the steak, drizzling it generously with the buttery sauce and garnish with freshly chopped parsley and chives. Serve immediately with sautéed spinach or roasted vegetables.