Melt 2 tablespoons of butter and 1 tablespoon of olive oil in a large skillet over medium heat. Add 2 sliced yellow onions and a pinch of salt. Cook for about 25 minutes until golden.

Peel and chop 2 Yukon Gold potatoes into uniform chunks. Cook in boiling salted water for 15-18 minutes until fork-tender, then drain and return to pot with butter and milk.

Combine ground beef, breadcrumbs, egg, Worcestershire sauce, Dijon mustard, minced garlic, onion powder, salt, and pepper in a bowl. Shape into 12 meatballs.

Heat 1 tablespoon of olive oil in a skillet. Sear meatballs for about 8 minutes until browned on all sides, then set aside.

In the same skillet, melt 2 tablespoons of butter and sauté 8 ounces of cremini mushrooms for about 5 minutes. Sprinkle in 2 tablespoons of flour, stirring to combine.

Gradually whisk in 2 cups of beef broth, 1 tablespoon of tomato paste, and 1 teaspoon of dried thyme. Stir until thickened.

Return meatballs to skillet and simmer for 12 minutes in the gravy. Stir occasionally.

If oven-safe, sprinkle 1 cup of shredded Swiss cheese over the meatballs and broil for 2-3 minutes until melted. Otherwise, transfer to baking dish to broil.

Serve by spooning mashed potatoes into bowls, topping with meatballs, gravy, caramelized onions, and garnish with parsley.