In a large bowl, combine the ground beef, breadcrumbs, seasoned salt, onion powder, garlic powder, Worcestershire sauce, chopped parsley, salt, and pepper. Mix gently with your hands until just combined — do not overwork. Roll into approximately 18-20 meatballs (about 1½ tablespoons each) and set aside on a plate.

In a medium bowl, whisk together the cold beef stock, tomato paste, brown mustard, Worcestershire sauce, garlic powder, and cornstarch until the cornstarch is fully dissolved. Set aside.

Heat 1 tablespoon of oil and 2 tablespoons of butter in a 12-inch skillet over medium-high heat. Add the meatballs in a single layer, leaving space between them. Brown on all sides, about 6-8 minutes total, turning every 2 minutes. The meatballs do not need to be cooked through at this stage. Transfer to a plate and drain most of the fat from the pan, leaving the browned fond on the bottom.

Reduce heat to medium. Melt 2 tablespoons of butter in the same skillet. Add the chopped onion and cook, stirring occasionally, until softened and translucent, about 3-4 minutes. Pour in the white wine and scrape up any browned bits from the bottom of the pan. Let the wine reduce by half, about 1 minute. Give the gravy mixture a final whisk, then pour it into the skillet. Stir continuously and cook until the gravy thickens, about 3-4 minutes.

Remove the skillet from the heat and swirl in the remaining 2 tablespoons of butter until melted and the gravy is glossy. Taste and adjust seasoning with salt as needed. Return the meatballs to the skillet, spoon the gravy over them, and simmer on medium-low for 3-4 minutes until the meatballs are cooked through and reach an internal temperature of 160°F (71°C). Garnish with fresh parsley and serve immediately over mashed potatoes with peas.