Mix the meatball base: In a large bowl, combine the ground beef, breadcrumbs, egg, onion, garlic, salt, and pepper. Mix gently with your hands just until combined; overmixing makes the meatballs tough.

Roll the meatballs: Use a small cookie scoop to portion the mixture, then roll into 1-inch balls, about 24 total. Keeping them uniform in size helps them cook evenly.

Brown in a hot skillet: Heat a large skillet over medium-high heat with a little oil. Add the meatballs in a single layer and brown on all sides, about 6-8 minutes total, until deep golden.

Remove and set aside: Transfer the browned meatballs to a plate. They don't need to be fully cooked through yet since they'll finish simmering in the glaze.

Build the glaze: In the same skillet, whisk together the cranberry sauce, soy sauce, rice vinegar, brown sugar, ketchup, and red pepper flakes if using. Scrape up the browned bits from the bottom of the pan as you stir.

Simmer the glaze: Bring the mixture to a gentle bubble over medium heat, stirring occasionally, until it thickens slightly, about 3-4 minutes.

Return meatballs to the pan: Add the browned meatballs back into the skillet, spooning the glaze over the top to coat them evenly.

Simmer until cooked through: Lower the heat and let everything simmer gently for 15 minutes, turning the meatballs occasionally, until the glaze turns thick and glossy and the centers read 160°F.

Garnish and serve: Sprinkle with chopped parsley or green onion and transfer to a warm serving dish with toothpicks for easy snacking.