Stack 3 slices of white bread and trim off the crust on the edges, and dice them. Transfer them to a large mixing bowl, pour in just enough milk to (1/3 cup) cover, and keep aside to let the bread soak in for 5-6 minutes. After soaked use a fork to coarsely mash up the breadcrumbs to a sauce-like consistency.
Heat 1 tbsp oil or butter in a pan, then add the chopped onion and garlic. Cook over medium heat, stirring occasionally until onions are light brown and fragrant.
Place the ground beef and pork into a large mixing bowl and add 1 tsp salt, 1 tsp black pepper, 1/4 cup finely chopped parsley, 1/4 cup parmesan cheese, 1 large egg, and sauteed onion and garlic.
Use your hands to mix the ingredients until evenly combined, avoid over-mixing the ingredients.
Gently form the mixture into 1 1/2-inch balls and set aside.
Preheat the air-fryer to 380°F.
Put shaped balls in a single-layer in the air-fryer basket.
Cook half the meatballs at 380°F until lightly browned and cooked through 12-14 minutes. You can also check with a meat thermometer, center of the meatball reaches 165 degrees.
Remove and keep warm. Repeat with remaining meatballs.
Serve warm meatballs as an appetizer or serve over rice, pasta, or noodles for a main dish.