Marinate Steak: In a mixing bowl, combine oil, Sriracha, pepper, and soy sauce, creating a flavorful marinade. Add steak strips, ensuring they're well-coated. Let it marinate for at least 30 minutes.

Sear Steak: Preheat a cast-iron skillet and melt unsalted butter until it foams. Place marinated steak strips in the skillet without overcrowding. Sear for 1-2 minutes until browned, then transfer to a plate.

Prepare Sauce: Reduce skillet heat, add more butter if necessary. Sauté shallot and garlic until fragrant. Stir in thyme, lemon juice, mustard, and beef stock, scraping browned bits from the skillet.

Finish and Serve: Simmer the sauce until reduced by half, about 3-5 minutes. Stir in remaining butter for a silky finish. Add seared steak back to warm through, plate, and garnish with chives.