In a large pot, combine the peeled and quartered Yukon Gold potatoes with the smashed garlic cloves. Cover with cold water by about an inch. Add 1 tablespoon of salt. Bring to a boil over high heat, then reduce heat to medium-low and simmer for 15-20 minutes, or until potatoes are fork-tender.

While potatoes are cooking, gently warm the milk (or half-and-half) in a small saucepan or microwave. Once potatoes are tender, drain thoroughly. Return the potatoes and garlic to the hot, empty pot. Add the cubed butter and warmed milk. Mash with a potato masher or ricer until smooth and creamy. Stir in chopped fresh chives and parsley. Season with salt and freshly ground black pepper to taste. Cover to keep warm.

In a large mixing bowl, gently combine the ground beef, panko breadcrumbs, egg, milk (or beef broth), finely minced yellow onion, minced garlic, 1 tbsp Worcestershire sauce, Dijon mustard, smoked paprika, onion powder, 1 tsp salt, and 1/2 tsp black pepper. Mix until just combined, being careful not to overmix, which can make the meatballs tough.

Roll the mixture into approximately 18-20 meatballs, about 1.5 inches in diameter.

Heat 2 tablespoons of olive oil in a large skillet (preferably oven-safe if you prefer to finish cooking in the oven, though stovetop works fine) over medium-high heat. Add the meatballs in a single layer (cook in batches if necessary to avoid overcrowding). Brown the meatballs on all sides, about 8-10 minutes total. They don't need to be cooked through, just nicely browned. Remove the browned meatballs from the skillet and set aside on a plate, leaving any drippings in the pan.

To the same skillet with drippings, add 2 tbsp unsalted butter. Once melted, add the sliced cremini mushrooms and finely chopped yellow onion. Sauté over medium heat for 5-7 minutes, until mushrooms are softened and have released their liquid, and onions are translucent.

Sprinkle 2 tbsp all-purpose flour over the mushrooms and onions. Stir constantly for 1-2 minutes, allowing the flour to cook and form a roux with the butter.

Gradually whisk in the 2 cups of beef broth, ensuring no lumps form. Add 1 tbsp Worcestershire sauce and 1 tsp soy sauce (if using). Bring the gravy to a gentle simmer, stirring occasionally, until it thickens to your desired consistency, about 5-7 minutes. Season with salt and black pepper to taste.

Return the browned meatballs to the skillet with the gravy. Reduce heat to low, cover, and let simmer for 10-15 minutes, or until the meatballs are cooked through and have absorbed some of the rich gravy flavors.

Serve the Salisbury steak meatballs and mushroom gravy generously over the warm garlic herb mashed potatoes. Garnish with extra fresh chives or parsley, if desired.