Marinate the Steak: In a bowl, combine the soy sauce (or coconut aminos), olive oil, Sriracha, and fresh cracked black pepper. Add the steak strips to the marinade, making sure they are well coated. Seal and chill in the refrigerator for a minimum of 30 minutes, or up to 2 hours for a bolder flavor.
Sear the Steak: Heat 1 tablespoon of unsalted butter in a large skillet over medium-high heat. Sear the steak strips in the hot skillet for 2-3 minutes on each side, or until they are browned and cooked to your desired doneness. Tent with foil to keep warm.
Make the Garlic Butter Mustard Sauce: In the same skillet, add 1 more tablespoon of unsalted butter. Add the finely chopped shallot and sauté for about 1-2 minutes until softened and fragrant. Add the minced garlic and a pinch of red chili pepper flakes.
Deglaze the Pan: Pour in the beef stock (or water) and lemon juice to deglaze the pan, scraping up any browned bits from the bottom of the skillet. Allow the liquid to simmer and reduce slightly, about 2-3 minutes.
Finish the Sauce: Stir in the Dijon mustard, minced thyme, and minced chives. Add the remaining tablespoon of butter and stir until melted and the sauce is smooth and glossy.
Combine and Serve: Return the seared steak strips to the skillet, tossing them in the sauce to coat evenly. Allow the steak to warm through in the sauce for about 1-2 minutes. Remove from heat.
Garnish and Serve: Transfer the steak strips to a serving platter, spooning any remaining sauce over the top. Garnish with fresh chopped parsley and serve immediately with your favorite sides such as mashed potatoes, steamed vegetables, or a fresh salad.