Prepare the Meatballs: In a large bowl, combine the ground beef, breadcrumbs, milk, egg, onion, garlic, salt, pepper, and smoked paprika. Mix gently with your hands until well combined, but do not overwork the mixture.
Shape the Meatballs: Roll the mixture into 1 to 1 ½-inch balls. You should get about 30–35 meatballs.
Pre-Brown the Meatballs (Optional but Recommended): Heat a skillet over medium-high heat with a bit of oil and brown the meatballs on all sides. This step locks in flavor and helps the meatballs hold their shape.
Prepare the Sauce: In a separate bowl, whisk together barbecue sauce, ketchup, brown sugar, apple cider vinegar, Worcestershire sauce, and Dijon mustard.
Layer in Slow Cooker: Pour a thin layer of sauce on the bottom of the slow cooker. Add the meatballs, then pour the remaining sauce over them. Gently stir to coat.
Cook: Low: 6 hours,High: 3–4 hour,Stir occasionally if possible to ensure even coating.
Serve: Once done, the sauce will be thick and glossy, and the meatballs will be tender and flavorful.