In a large skillet, melt 1 tablespoon butter over medium heat. Add onions and cook until soft and translucent.
In the bowl of a food processor, pulse bread pieces until small crumbs form. Add in ground pork, cooked onions, salt, and pepper, and pulse until meat is chopped and mixture clumps together. (Use a mixing bowl and your hands to mix if you don't have a food processor.)
Form mixture into small meatballs, rolling in your hands or using a cookie scoop.
Melt 1 tablespoon butter in the skillet over medium-low heat, then add meatballs. Cook, turning and stirring, about 15 minutes, until meatballs are browned and cooked through. Remove from pan.
Add 1 tablespoon butter to skillet and let melt over medium-low, then whisk in flour, scraping up browned bits. Whisk in milk.
Continue to whisk until gravy thickens (this will happen quickly), season with salt and pepper to taste, then add back in meatballs and stir to coat. Top with parsley and serve.