Preheat the oven to 375°F (190°C). Line a baking sheet with parchment or lightly oil it.

Caramelize the onion first: thinly slice the onion, and cook slowly in a skillet over medium-low heat with a little oil or butter until deeply golden and sweet (about 25–30 minutes). Cool slightly.

In a large bowl, combine the ground beef, breadcrumbs, Parmesan, egg, cooled caramelized onion, garlic powder, onion powder, salt, and pepper. Mix gently until just combined.

Shape the mixture into even meatballs (about 1.5 inches in diameter) and place them on the prepared baking sheet.

Bake meatballs for 20–25 minutes until they register 160°F (71°C) with an instant-read thermometer.

Meanwhile, in a saucepan, combine the beef broth and Worcestershire sauce, and bring to a gentle simmer.

Transfer the baked meatballs into the simmering broth and let them steep for 4–5 minutes.

Move the meatballs to an oven-safe serving dish, spoon a little of the broth over them, and top evenly with shredded mozzarella.

Return to the oven for 3–6 minutes, or broil briefly, until the cheese is melted and slightly golden.

Garnish with chopped parsley and serve hot. Offer crusty bread to mop up the broth or serve over mashed potatoes.