Sear the beef: Heat a large pot over medium-high heat. Add 2 tablespoons (28g) of oil to the pot. When the oil starts to shimmer, add only ½ of the beef to not overcrowd the pan (stew meat or steak). Season with salt and pepper and sear on all sides until a brown crust forms, about 4-5 minutes. Note: If your pan doesn't seem hot enough, increase to high heat. Remove from the pot to a plate and continue with the rest of the beef. Set aside.
Reduce the heat to medium and add the butter, diced onion, garlic, and mushrooms. Season with salt and pepper. Cook, stirring occasionally, until the mushrooms are soft, about 8 minutes. Stir in the Worcestershire sauce, dijon mustard, and beef broth.
For slow on the stove top (see notes for slow cooker or Instant pot instructions): Stir in the seared stew meat with any accumulated juices to the pot. Scrape any browned bits from the bottom of the pan with a wooden spoon. Bring to a boil, then reduce to a simmer. Cover with a lid and let simmer for 1 hour, stirring occasionally, or until the stew meat is tender.
For fast: Thicken the sauce, then add the steak back to the pot. Proceed with adding the sour cream and serving.
Thickening the sauce: In a small bowl, combine the remaining ½ tablespoon (7g) of oil with the Xanthan gum (this prevents clumping). Stir into the stroganoff and continue to simmer for a few minutes more to thicken. Note: the sauce will continue to thicken with the addition of the sour cream and as it cools. See notes for an alternative thickener.
Temper the sour cream before adding: Before adding the sour cream, it needs to be tempered so it doesn't break. In a small bowl, combine the sour cream with about ½ cup (100ml) of the hot broth from the pot. Stir well, then stir it into the stroganoff. Season the stroganoff with salt and pepper to taste. Remove from the heat and serve.