Prep your pan and oven: Heat the oven to 400°F (200°C). Line a rimmed baking sheet with foil and set a wire rack on top if you have one.
Lightly oil the rack or the foil.
Soak the crumbs: In a large bowl, mix breadcrumbs and milk. Let them sit 2–3 minutes until the crumbs are moistened. This step helps keep the meatballs tender.
Build the meat mixture: Add egg, grated onion, garlic, Worcestershire, salt, pepper, paprika, and parsley.
Stir until combined, then add the ground meats. Gently mix with your hands until just combined. Do not overwork or the meatballs can get dense.
Shape the meatballs: Form 1 to 1 1/4-inch balls (about 1 heaping tablespoon each). You should get 30–36 meatballs.
Lightly oil your hands if the mixture is sticky.
Bake until browned: Arrange meatballs on the pan. Bake 12–15 minutes, or until lightly browned and cooked through (internal temp 160–165°F/71–74°C). They’ll finish in the sauce, so don’t overbake.
Make the sauce: While the meatballs bake, whisk ketchup, jelly, vinegar, soy sauce, brown sugar, Dijon, garlic powder, onion powder, and hot sauce in a large skillet or pot.
Warm over medium heat until the jelly melts and the sauce is smooth and glossy.
Combine and simmer: Add the baked meatballs to the sauce. Gently toss to coat and simmer 5–10 minutes so flavors meld. Adjust sweetness, tang, or heat to taste with more brown sugar, vinegar, or hot sauce.
Serve and keep warm: Transfer to a slow cooker on the Warm or Low setting for parties, or serve straight from the skillet.
Garnish with chopped herbs and set out toothpicks.