Soak the breadcrumbs. In a large bowl, combine breadcrumbs and milk.
Stir and let sit for 5 minutes until the mixture becomes a soft paste (a panade). This keeps your meatballs tender.
Mix the meatball base. Add ground beef, ground pork, eggs, 1/2 cup Parmesan, half the minced garlic, parsley, 1 teaspoon salt, and 1/2 teaspoon black pepper to the bowl. Gently mix with your hands until just combined.
Don’t overwork it.
Shape the meatballs. Roll into golf ball–sized portions (about 1.5 inches). You should get around 20–24 meatballs. Place them on a lined baking sheet or plate.
Brown the meatballs. Heat 2 tablespoons olive oil in a large, wide pot or Dutch oven over medium heat.
Sear meatballs in batches, turning to brown most sides, about 5–7 minutes total per batch. Transfer to a plate. Don’t worry if they’re not cooked through yet.
Sauté the aromatics. In the same pot, add another tablespoon of olive oil if needed.
Add the chopped onion with a pinch of salt and cook until soft and translucent, 5–7 minutes. Stir in the remaining garlic and the red pepper flakes; cook 30 seconds until fragrant.
Build the sauce. Stir in tomato paste and cook for 1 minute to toast it. Pour in the crushed tomatoes.
Add 1 teaspoon salt, a few grinds of pepper, and the sugar if using. Bring to a gentle simmer.
Simmer with meatballs. Nestle the browned meatballs into the sauce, along with any juices. Simmer uncovered on low for 25–30 minutes, gently stirring occasionally, until meatballs are cooked through and sauce thickens slightly.
Cook the pasta (optional). If serving with pasta, cook it in well-salted boiling water until al dente.
Reserve 1/2 cup pasta water.
Finish and adjust. Taste the sauce. Add more salt, pepper, or a pinch of sugar if needed. Stir in the remaining 1/4 cup Parmesan for body.
If serving with pasta, toss pasta with a ladle of sauce and a splash of pasta water for cling, then plate with meatballs and more sauce.
Serve. Drizzle with a little olive oil, shower with extra Parmesan, and top with torn basil if you like.