Mix breadcrumbs and milk in a small bowl. Let sit 5 minutes until thick and paste-like.

In a large bowl combine ground beef, breadcrumb mixture, eggs, Parmesan, parsley, salt, pepper, and garlic. Mix gently just until combined. Do not overwork. Form into 1½–2 inch meatballs (about 30–34).

Set multi-cooker to Sauté (Medium). Add olive oil and brown meatballs in batches, turning to color 2–3 sides, about 6–8 minutes per batch. Transfer to a plate. Do not wipe out the pot.

Still on Sauté, add olive oil if needed. Add onion with a pinch of salt and cook 5–6 minutes until softened. Add garlic and cook 30 seconds. Stir in tomato paste and cook 1–2 minutes until slightly darkened. Turn off Sauté. Add whole tomatoes (crushing by hand), crushed tomatoes, salt, and Parmesan rind. Stir, scraping up browned bits. Nestle meatballs into the sauce.

Set to Slow Cook – LOW for 6–7 hours or HIGH for 3–4 hours. Keep covered. Add torn basil at the end and adjust salt. If sauce feels thin, remove lid for last 30 minutes on HIGH to reduce slightly.

Brown meatballs in a skillet and transfer to a plate. In the same skillet, cook onion in olive oil until softened, add garlic for 30 seconds, then stir in tomato paste and cook until slightly darkened. Transfer to slow cooker. Add whole tomatoes (crushed), crushed tomatoes, salt, and Parmesan rind. Stir well, nestle in meatballs, cover, and cook LOW 6–7 hours or HIGH 3–4 hours.

Boil spaghetti in heavily salted water until al dente. Transfer pasta to a skillet with 1–2 ladles of sauce and toss 1–2 minutes so it absorbs flavor. Plate with meatballs, more sauce, and Parmigiano.