Prep the panade: In a small bowl, combine the breadcrumbs with the milk and let sit for 5 minutes until a thick paste forms.
Mix the meatball base: In a large bowl, combine the ground beef, ground pork, panade, egg, Parmesan, garlic, parsley, salt, pepper, oregano, and crushed fennel seeds. Use your hands to gently mix just until everything is combined. Do not overwork.
Form the meatballs: Lightly oil your hands, then roll the mixture into 1½-inch balls (about the size of a golf ball). You should get about 20 meatballs. Place them on a rimmed baking sheet.
Brown the meatballs: Heat olive oil in a Dutch oven over medium-high heat. Working in two batches, add the meatballs and cook until deeply browned on all sides, about 6–8 minutes per batch. Do not crowd the pot. Transfer to a plate.
Start the sauce: Reduce heat to medium. Add the chopped onion to the same pot and cook until softened and golden, about 5 minutes. Add the garlic and cook 30 seconds. Add the crushed tomatoes, salt, and sugar if needed. Scrape up any browned bits from the bottom of the pot.
Simmer the meatballs in sauce: Nestle the meatballs back into the pot, along with any accumulated juices. Add the basil sprigs. Bring to a gentle simmer, then cover partially and cook for 45 minutes to 1 hour, stirring gently once or twice. The sauce will thicken and the meatballs will become tender.
Serve: Remove the basil stems. Spoon over pasta, polenta, or pile onto crusty rolls. Top with grated Parmesan and a drizzle of good olive oil.