Place the peeled and quartered Yukon gold potatoes and 4 smashed cloves of fresh garlic in a large pot. Cover with cold salted water and bring to a boil. Reduce heat and cook until fork-tender, about 15-20 minutes.

Drain the potatoes, remove the garlic. Return potatoes to the pot. Heat the half and half and cut butter into cubes. Add butter and cream to hot potatoes. Mash until creamy, then stir in chopped fresh parsley, salt, and pepper. Keep warm, covered.

In a small bowl, combine the Panko breadcrumbs and milk. Let sit for 5 minutes.

In a large mixing bowl, combine the ground beef, soaked breadcrumbs, minced brown onion, minced fresh garlic, egg, Worcestershire sauce, salt, pepper, and mustard powder. Mix gently until just combined.

Roll the mixture into 1.5-inch balls, making about 18-20 meatballs.

Heat 1 tablespoon olive oil and 1 tablespoon butter in a large skillet over medium-high heat. Sear the meatballs until golden brown on all sides, about 5 minutes. Remove and set aside.

Add remaining 2 tablespoons butter to the pan. Scrape up brown bits, add sliced mushrooms, cook until browned, about 5-7 minutes. Add any remaining minced onion if needed, cook until translucent.

Sprinkle flour over vegetables, cook for 1 minute. Slowly whisk in beef broth, Worcestershire sauce, ketchup, crushed bouillon, and thyme.

Bring to a boil, reduce to simmer. Return meatballs to pan, cover, simmer for 15-20 minutes until cooked through and gravy thickened.

Dollop mashed potatoes onto plates, top with meatballs and gravy, garnish with fresh parsley.