Cook the egg noodles according to the package directions. (I usually start boiling the water after searing the meat in step 3.)
Place the fillets on a cutting board, cover with wax paper or parchment, and pound with a meat mallet until ½ inch thick. Season them with a generous amount of salt and pepper. Slice against the grain into ½-inch strips, then cut crosswise into bite-sized pieces and set aside.
Heat a skillet over medium-high heat. Melt a tablespoon of butter, and once it’s foaming, add the beef and sear for 30 seconds on each side—no more. Use a slotted spoon to transfer the meat to a paper towel-lined plate; set aside.
Add the remaining butter to the skillet. Add the onions and sauté until soft (about 5 minutes).
Add the thyme and mushrooms, and sauté until the mushrooms are tender (5 to 7 minutes). Stir in the garlic and sauté just until fragrant (about 1 minute).
Pour the wine into the skillet. Bring it to a low simmer, scraping up any browned bits from the bottom of the pan as it cooks down.
Meanwhile, whisk together the beef broth, mustard, Worcestershire, and flour in a small bowl until smooth, stirring in the Better Than Bouillon if you’re using it. Add this to the skillet and let it simmer for about 5 minutes, until thickened.
Stir in the sour cream until it melts into the sauce, creating a nice and creamy gravy. Taste and adjust seasoning as needed. (See my recipe notes about tempering the sour cream to avoid curdling—especially if using a low-fat variety of dairy.) Bring the seared beef back to the pan and stir to coat. From here, you can toss the cooked egg noodles right in with everything or plate the noodles and spoon the stroganoff over top.