Preheat your oven to 375°F (190°C). Grease a 2.5-quart white casserole dish with butter or non-stick spray. Bring a large pot of salted water to a boil for the egg noodles.

In a large skillet over medium-low heat, melt the 2 tbsp butter with the olive oil. Add the thinly sliced onions and sprinkle with the sugar. Cook, stirring occasionally, for 12-15 minutes until the onions are deeply golden brown, soft, and caramelized. Transfer to a bowl and set aside.

In the same skillet over medium-high heat, cook the ground beef, breaking it up with a spoon, until no pink remains, about 6-8 minutes. Drain any excess fat. Meanwhile, cook the egg noodles in the boiling water according to package directions for al dente (usually 6-7 minutes). Drain.

In a large mixing bowl, whisk together the cream of mushroom soup, beef broth, sour cream, Worcestershire sauce, thyme, garlic powder, pepper, and salt until smooth and well combined.

Add the drained egg noodles, cooked ground beef, and caramelized onions to the bowl with the sauce. Gently fold everything together until evenly coated. Stir in 3/4 cup of the shredded Gruyère cheese.

Transfer the mixture to the prepared casserole dish and spread it into an even layer. Sprinkle the remaining 1/4 cup Gruyère and the 1/2 cup Swiss cheese evenly over the top. In a small bowl, mix the panko breadcrumbs with the 1 tbsp melted butter and sprinkle over the cheese.

Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly around the edges and the top is golden brown. If desired, broil for the final 1-2 minutes for extra browning (watch closely).

Remove from the oven and let the casserole rest for 5-10 minutes. This allows the sauce to thicken slightly. Garnish with chopped fresh parsley. Serve hot directly from the white casserole dish with a wooden spoon.