Combine the ground beef, finely diced yellow onion, breadcrumbs, minced garlic, egg, ketchup, mustard, Worcestershire sauce, salt, pepper, and chopped parsley in a large bowl. Mix until all the ingredients in your bowl until they are evenly incorporated.

Using clean hands, a tablespoon, or an ice cream scoop, shape the meat mixture into meatballs, they should be about 1-2 inches in diameter. Set your homemade meatballs aside in a single layer. I like to put mine on a pan.

In a skillet, heat 1 tablespoon olive oil or butter over medium-high heat. Add the meatballs and cook them until they are browned on all sides and cooked through. This will take about 10-12 minutes. They are ready when they reach an internal temperature of 160°F

Heat 1-2 tablespoons of oil in a large skillet over medium heat, and then add the remaining onions. Cook them until they are soft and caramelized. This should take about 8-10 minutes.

In the same skillet, melt the remaining oil or butter over medium heat. Sprinkle the flour over the oil or melted butter and whisk continuously until the mixture turns golden brown and fragrant, about 2-3 minutes.

Slowly pour in the beef broth and add Worcestershire sauce while whisking constantly. Continue whisking until the gravy thickens and reaches your desired consistency. Season your flavorful sauce with salt and pepper to taste. For a lighter and creamier gravy, add heavy cream or sour cream.

Return the caramelized onions to the skillet with the gravy and stir to combine.

Gently place the cooked meatballs into the skillet and coat them with the creamy gravy sauce. Simmer the meatballs in the rich gravy for an additional 3-5 minutes to allow the flavors to meld together.