Combine flour, garlic powder, salt, black pepper, and paprika in a shallow bowl. Trim any excess fat from the beef chuck pieces, then toss them thoroughly in the flour mixture until all pieces are evenly coated. Set the seasoned beef aside on a plate. This coating will help create a flavorful crust and thicken the braise.

Heat vegetable oil and butter in a large skillet over medium-high heat until the butter foams. Working in batches if needed, brown the floured beef on all sides, about 8-10 minutes total—don't overcrowd the pan. Transfer the browned beef to a 9x13 inch baking dish. In the same skillet, add diced onion and sauté for 3-4 minutes until softened, then add mushroom slices and cook for another 3-4 minutes until they begin to release their moisture. Pour this mixture into the baking dish with the beef.

Add thyme sprigs and beef broth to the baking dish and stir gently to combine. Cover tightly with foil and bake at 300°F for 2 hours. This low temperature allows the tough beef chuck to break down slowly and become incredibly tender. I like to stir it halfway through for even cooking, though it's not essential.

While the beef braises, bring a pot of salted water to boil and cook the egg noodles 2 minutes less than the package directions suggest—this ensures they won't become mushy when baked. Drain and set aside. In a small bowl, whisk together sour cream, beef gravy, and cornstarch until smooth. The cornstarch helps thicken the sauce and prevents the sour cream from breaking during cooking.

Remove the beef from the oven and increase temperature to 350°F. Remove and discard the thyme sprigs, then stir in the cooked noodles from Step 4 and the sour cream mixture from Step 4. Mix gently until everything is evenly coated and combined. Cover with foil and bake for 15 minutes until the sauce is heated through and bubbles gently at the edges.

Remove from oven and let rest for 5 minutes—this allows the sauce to set slightly and makes serving easier. Sprinkle with fresh parsley just before serving. For the best results, I always use full-fat sour cream and good quality butter; these small choices make a noticeable difference in the final texture and richness of the stroganoff.