Set a large, flat-sided skillet or Dutch oven over medium-high heat. Once the pan is hot, add 1 tablespoon of oil, swirling to coat the pan. When the oil is shimmering but not smoking, add the mushrooms. Leave them alone for 30 seconds without stirring, then season with salt and pepper, shaking the pan to toss the mushrooms. Continue cooking, stirring occasionally, until soft and caramelized, 3-4 minutes. Transfer to a medium-sized bowl.
Season the beef with salt and pepper. Add the remaining tablespoon of oil + 1 tablespoon of the butter to the pan, swirling to coat. Add the beef to the pan in a single layer (cook in batches to avoid overcrowding). Sear for 30 seconds on high heat. Flip the meat with tongs and sear for an additional 30 seconds, then transfer to the bowl with the mushrooms. Repeat with the remaining meat. If the pan ever gets too smoky, take it off the heat for a moment. The meat can be slightly undercooked; it will finish cooking in the sauce at the end.
Turn the heat down to medium-low. Add the remaining tablespoon of butter along with the onions and a pinch of salt. Stir with a spatula or wood spoon, scraping up the brown bits on the bottom of the pan as they detach and incorporating them back into the onions. Continue stirring until the onions are soft and the brown bits are incorporated, 2-3 minutes.
Stir in the flour and continue cooking for an additional 2-3 minutes. Whisk in the stock, wine, and juices from the bowl with the beef and mushrooms (a fine-mesh strainer makes this easier, if you have one). Increase the heat to medium-high and bring to a boil, whisking until the sauce thickens slightly, 2-3 minutes.
Reduce the heat to medium-low. Add a few ladles of hot liquid into the sour cream, whisking to combine, then add the sour cream to the sauce (don’t skip this step; the sour cream can curdle if added directly to hot liquid). Add the mushrooms and beef back to the pan; heat until warmed through; 1-2 minutes. Taste and add salt and pepper if needed (or a splash of soy sauce instead of salt).
Serve Beef Stroganoff over buttered egg noodles, topped with chopped parsley. Leftovers will keep in an airtight container in the refrigerator for up to 5 days.
Beef tenderloin will produce more tender results, but can be quite a bit more pricey. I used sirloin. You can either prepare the egg noodles in advance and then reheat them with butter for serving, or you can have salted water boiling when you start cooking. Start cooking the pasta around the same time you add the onions to pan.