Step 1: Season the beef cubes generously with salt and pepper. Heat 1 tablespoon of olive oil in a large Dutch oven or oven-safe pot over medium-high heat. Sear the beef in batches until browned on all sides. Remove the beef and set aside.
Step 2: Reduce heat to medium-low, add the remaining 1 tablespoon of olive oil and 2 tablespoons of butter to the pot. Add the thinly sliced onions and the optional granulated sugar. Cook slowly, stirring occasionally, for 30-45 minutes, until the onions are deeply caramelized and jammy.
Step 3: Add the minced garlic to the pot with the caramelized onions and cook for 1 minute until fragrant. Sprinkle the flour over the onions and stir well to coat, cooking for 2 minutes to cook out the raw flour taste.
Step 4: Gradually whisk in the beef broth and red wine, scraping up any browned bits from the bottom of the pot. Stir in the Worcestershire sauce, fresh thyme, and bay leaf. Bring the mixture to a simmer.
Step 5: Return the seared beef to the pot. Cover and transfer to a preheated oven at 325°F (160°C). Braise for 2-2.5 hours, or until the beef is fork-tender. Remove the bay leaf before proceeding.
Step 6: While the beef is braising, cook the egg noodles according to package directions until al dente. Drain and set aside.
Step 7: Preheat your oven to 375°F (190°C). In a large bowl, combine the cooked egg noodles with the braised beef and onion mixture. Mix well to ensure everything is evenly coated.
Step 8: Transfer the noodle and beef mixture to a 9x13 inch baking dish. Sprinkle the grated Gruyere and mozzarella cheeses evenly over the top.
Step 9: Bake for 20-25 minutes, or until the cheese is melted, bubbly, and lightly golden brown. If desired, broil for the last 2-3 minutes for a deeper golden crust, watching carefully to prevent burning.
Step 10: Let the casserole rest for 10 minutes before serving. Garnish with fresh chopped parsley, if using, and serve hot.