Step 1: Caramelizing the Onions

1. Heat a large skillet over medium heat. Add butter and olive oil.
2. Once the butter melts, add the sliced onions and stir to coat them in the fat.
3. Cook the onions slowly for about 25-30 minutes, stirring occasionally. To enhance caramelization, sprinkle the sugar after the first 10 minutes.
4. Once the onions turn a deep golden brown, transfer them to a bowl and set aside.

Step 2: Cooking the Beef

1. In the same skillet, add a little more olive oil if needed.
2. Season the thinly sliced beef with salt, pepper, and garlic powder.
3. Increase heat to medium-high and sear the beef for 2-3 minutes per side until browned.
4. Add Worcestershire sauce and thyme, then stir to coat the beef.
5. Remove the beef from the skillet and set aside with the onions.

Step 3: Making the Sauce

1. In the same skillet, sprinkle the flour over the remaining beef drippings and whisk continuously for about 1 minute.
2. Slowly pour in the beef broth, whisking constantly to prevent lumps.
3. Let the mixture simmer for about 5 minutes until it thickens slightly.
4. Reduce heat to low and stir in the heavy cream, Dijon mustard, and shredded Gruyère cheese.
5. Continue stirring until the cheese melts and the sauce is smooth.

Step 4: Cooking the Noodles

1. While preparing the sauce, bring a large pot of salted water to a boil.
2. Cook the egg noodles according to package instructions until al dente.
3. Drain and toss the noodles with butter to prevent sticking.

Step 5: Combining Everything

1. Return the caramelized onions and cooked beef to the skillet with the sauce.
2. Stir well to coat everything in the creamy, cheesy sauce.
3. Add the cooked noodles and toss gently until well combined.

Step 6: Garnishing and Serving

1. Sprinkle chopped fresh parsley over the top for a pop of color and freshness.
2. If desired, add extra shredded Gruyère cheese and allow it to melt into the dish.
3. Serve immediately while warm and creamy.