Sear the Beef: Heat a large pot over medium-high heat. Add 2 tablespoons of oil. Season your beef with salt and pepper. Brown half of the beef at a time for about 4-5 minutes until a nice crust forms. Remove the browned beef and set aside.
Sauté the Veggies: Reduce heat to medium. Add butter, diced onion, minced garlic, and sliced mushrooms to the pot. Cook for about 8 minutes, stirring occasionally, until the vegetables are tender.
Build the Sauce: Stir in the Worcestershire sauce, Dijon mustard, and beef broth.
Cook the Beef:
For Slow-Cooked Stroganoff (using stew meat): Return the seared beef to the pot. Bring the mixture to a boil, then reduce to a gentle simmer. Cover and cook for 1 hour, or until the beef is deliciously tender.
For Fast Stroganoff (using thin-sliced steak): Proceed directly to thickening the sauce, then add the cooked steak back into the pot.
Thicken the Sauce: In a small bowl, whisk the remaining ½ tablespoon of oil with the xanthan gum until smooth. Stir this mixture into the simmering sauce and cook for a few more minutes until it visibly thickens.
Temper & Finish with Sour Cream: To prevent curdling, temper the sour cream: in a separate small bowl, mix the sour cream with about ½ cup of the hot sauce from the pot. Stir well, then gently fold this tempered sour cream into the stroganoff. Taste and adjust seasoning with salt and pepper. Remove from heat and serve immediately.