Step 1: In a large bowl, combine the breadcrumbs and milk. Let it sit for 5-10 minutes to allow the breadcrumbs to fully absorb the liquid and create a panade.
Step 2: Add the ground beef, ground pork, grated Parmesan cheese, chopped parsley, minced garlic, beaten egg, salt, black pepper, and red pepper flakes (if using) to the bowl with the breadcrumb mixture. Use your hands to gently combine all ingredients until just incorporated. Be careful not to overmix, as this can lead to tough meatballs.
Step 3: Using a tablespoon or a small cookie scoop, gently roll the mixture into golf-ball-sized meatballs (about 1.5 to 2 inches in diameter). Place the formed meatballs on a baking sheet lined with parchment paper.
Step 4: Heat 2 tablespoons of olive oil in a large, heavy-bottomed pot or Dutch oven over medium-high heat. Once hot, carefully add the meatballs in batches, ensuring not to overcrowd the pot. Brown them on all sides until a nice crust forms. You're not cooking them through, just developing flavor on the exterior. Remove the browned meatballs and set them aside.
Step 5: In the same pot (adding a touch more olive oil if needed), add the finely chopped onion and sauté over medium heat until softened and translucent, about 5-7 minutes. Stir in the crushed tomatoes, tomato sauce, pinch of sugar, and the bay leaf. Bring the sauce to a gentle simmer, stirring occasionally.
Step 6: Carefully add the browned meatballs back into the simmering sauce. Ensure they are mostly submerged. Reduce the heat to low, cover the pot, and let the meatballs simmer gently for at least 45 minutes to 1 hour, or even longer for deeper flavor development and tenderness. The longer, the better!
Step 7: Before serving, remove and discard the bay leaf. Garnish with fresh basil and extra grated Parmesan cheese. Serve hot with your favorite pasta, on sub rolls, or as desired.