Place the peeled and cubed Yukon Gold potatoes in a large pot of salted water. Bring to a boil and cook for 15-20 minutes until fork-tender.

In a large bowl, gently combine the ground beef, panko breadcrumbs, beaten egg, minced onion, minced garlic, Worcestershire sauce, A.1. sauce, salt, and black pepper. Use your hands like a gentle massage to avoid making the meat tough.

Form the mixture into small meatballs, about 1-1.5 inches in diameter. Heat 2 tbsp vegetable oil in a large skillet over medium-high heat. Sear the meatballs until a deep, dark brown crust forms on all sides, about 5-7 minutes. Remove and set aside; they don't need to be fully cooked yet.

In the same skillet, melt 2 tbsp butter. Add the sliced mushrooms and sauté until browned and their moisture is released, about 5 minutes.

Sprinkle 2 tbsp flour over the mushrooms to form a roux. Gradually whisk in the beef broth, stirring constantly to thicken. Add chopped thyme and 1 tbsp of the chopped parsley. Bring to a simmer.

Place the seared meatballs back into the gravy. Cover and simmer for 10-12 minutes until the meatballs are cooked through and tender.

Drain the potatoes and return to the pot. Add 1/4 cup butter (if not in gravy), half & half, garlic powder, Italian seasoning, and the remaining 1 tbsp parsley. Mash until smooth and fluffy.

Spoon mashed potatoes into bowls, top with meatballs, and ladle on the mushroom gravy. Serve hot.