Prepare the beef. Thinly slice the top sirloin, boneless ribeye, beef tenderloin (filet mignon), or filet mignon tips against the grain into bite-sized strips. Pat the beef dry with paper towels and season with salt and pepper.
Sear the steak. Melt 1–2 tablespoons of butter in a large skillet over medium-high heat. Working in batches, sear the beef for 1–2 minutes per side until browned. Avoid overcrowding the pan so the steak develops a good sear. Transfer the beef to a plate; it will finish cooking later. If choosing to use a beef roast, click the link in the ingredient list for how I cook a delicious roast.
Sauté the onion and garlic. Reduce the heat to medium and add the remaining butter into the large pan. Add the diced onion and cook for 4-5 minutes, stirring occasionally, until softened. Add the minced garlic and cook for another 30 seconds, until fragrant.
Make the gluten-free roux. Sprinkle the white rice flour over the onion mixture and stir continuously for about 1 minute to coat the onions and cook out the raw flour taste.
Build the sauce. Slowly whisk in the beef broth, scraping up any browned bits from the bottom of the skillet. Stir in the Worcestershire sauce and Dijon mustard. Simmer for 3–5 minutes, stirring occasionally, until the sauce begins to thicken over medium-low heat.
Add the cream. Reduce the heat to medium-low and stir in the heavy cream. Let the sauce gently simmer for another 2–3 minutes until smooth and creamy. Avoid boiling the sauce.
Return the beef to the skillet. Add the seared steak and any accumulated juices back to the sauce. Simmer gently for 1–3 minutes, or just until the beef reaches your preferred doneness. Thinner slices and more tender cuts may need less time.
Finish with sour cream. Remove the skillet from the heat or reduce it to very low. Stir in the sour cream until fully incorporated. Taste and adjust the salt and pepper as needed.
Cook the noodles. While the sauce is simmering, prepare the Jovial gluten-free egg noodles according to the package directions. Drain well.
Serve. Spoon the creamy beef stroganoff over the cooked gluten-free egg noodles. Garnish with fresh parsley, if desired, and serve immediately. Store leftovers in an airtight container.