Heat the olive oil in a large skillet over medium heat. Add the sliced onions and cook, stirring often, until soft and golden, about 20 to 25 minutes. Transfer the onions to a plate.

Add the chopped mushrooms to the same skillet. Cook until their liquid has evaporated and the mushrooms are tender, about 6 to 8 minutes. Set aside and let them cool slightly.

Heat the oven to 375°F. In a large bowl, combine the ground turkey, egg, panko, milk, Worcestershire sauce, thyme, salt, and pepper. Add the cooled mushrooms and half of the cooked onions. Mix gently until combined, then shape into a loaf in a baking dish.

Bake the meatloaf for 35 minutes. Scatter the Gruyère over the top and bake for another 10 to 15 minutes, until the center reaches 165°F. Let it rest for 10 minutes before slicing.

While the meatloaf bakes, return the remaining onions to the skillet. Add the beef broth and simmer for 5 to 7 minutes, stirring to loosen any browned bits, until slightly reduced.

Cook the egg noodles in a pot of salted boiling water according to the package directions. Drain, then toss with the butter and parsley.

Serve slices of meatloaf over the buttered egg noodles with the warm onion and mushroom broth spooned over the top.