Prepare the beef. Pat the sirloin steak dry with paper towels. Slice it into strips approximately 1/2 inch thick, or cut it into 1-inch cubes if preferred. Season evenly with the seasoned salt and black pepper.
Coat the steak. Sprinkle 1 tablespoon of all-purpose flour over the seasoned beef and toss until the pieces have a light, even coating. Shake away any loose excess flour.
Brown the beef. Warm the olive oil in a large skillet or Dutch oven over medium-high heat. Working in small batches, sear the beef for approximately 1 minute per side. The exterior should be lightly browned while the center remains undercooked. Transfer each finished batch to a plate and reserve.
Sauté the onion. Lower the heat to medium and melt the salted butter in the same skillet. Add the diced yellow onion and cook for about 4–5 minutes, or until softened.
Cook the mushrooms and garlic. Add the sliced mushrooms and minced garlic to the softened onion. Continue cooking for approximately 4 minutes, stirring occasionally, until the mushrooms become tender.
Form the sauce base. Sprinkle the remaining 2 tablespoons of all-purpose flour over the mushroom mixture. Stir thoroughly and cook for 1 minute to remove the raw flour flavor.
Add the liquids and seasonings. Gradually pour in the beef broth, then stir in the Worcestershire sauce, Dijon mustard, and dried thyme. Bring the mixture to a boil while stirring to keep the sauce smooth and free of lumps.
Thicken the sauce. Reduce the heat and simmer the sauce uncovered for 5 minutes, allowing it to thicken.
Return the beef to the skillet. Add the browned steak along with any juices that accumulated on the plate. Cook for another 2 minutes, only until the beef is heated through.
Finish with sour cream. Remove the skillet from the heat. Immediately stir in the sour cream until fully incorporated and the sauce is smooth and creamy. Taste and add additional salt or pepper if desired.
Serve. Spoon the finished beef stroganoff over cooked egg noodles. Pasta or mashed potatoes may also be used as the base.