Bring a pot of salted water to a boil. Cook the egg noodles until tender, about 7 to 9 minutes. Drain and set aside.

In a 12-inch oven-safe skillet over medium-high heat, cook the ground beef, breaking it up, until browned, 6 to 8 minutes. Season with the salt and pepper; transfer to a plate, leaving fat in the pan.

Reduce heat to medium and melt the butter in the skillet. Add sliced onions and cook, stirring often, until translucent and golden at edges, 8 to 10 minutes. Stir in garlic until fragrant, about 30 seconds.

Sprinkle flour over onions and stir for 1 minute. Pour in beef broth, Worcestershire, and thyme, scraping up browned bits. Simmer until slightly thickened, 3 to 4 minutes. Remove from heat, then stir in sour cream and 1 cup Gruyère until melted. Return beef and fold in noodles until coated.

Preheat oven to 375°F. Smooth mixture into an even layer and top with remaining 1 cup Gruyère. Bake until cheese is melted and bubbling, 18 to 22 minutes. Let rest 5 minutes, garnish with parsley, and serve.