I recommend prepping all ingredients before getting started to make it easier. The meatballs and sauce both use minced garlic, so be sure to mince it all at once (I use a garlic press to make this go fast). Add the canned tomatoes (with juices) to a large bowl and use your hands to break the whole tomatoes up.
To a soup pot, add the olive oil, butter, and onion and sauté over medium heat for 5 minutes.
Stir in the garlic and cook for 30 seconds.
Add in the tomatoes, Italian seasoning, oregano, sugar, and red pepper flakes. Give it a good stir. Once the sauce starts to bubble, cover the pot with the lid slightly open and reduce the heat and simmer for 30 minutes (adjust between medium-low and low heat as needed to maintain a constant simmer).
While the sauce is cooking, make the meatballs. Add the breadcrumbs and milk to a fairly large prep bowl. Mix/mash together until you've got a paste.
Add the remaining meatball ingredients to the bowl. Using your hands, mix it together and then form about 18 (1 ½-inch) meatballs. Don't over-mix or pack them too tightly, and use a delicate touch. Place the meatballs on a couple of plates or a parchment paper lined baking sheet as you go along for easy clean-up.
Give the sauce a stir, then place the meatballs in the pot and gently arrange them so they're submerged in the sauce. Continue simmering (lid slightly open) for another 35-45 minutes or until they're cooked through (165F). You may need to increase the heat to get it going again, then reduce the heat and adjust as needed to maintain a simmer. If you have a strong preference, feel free to brown the meatballs in a skillet ahead of adding them to the sauce, but our kitchen tested both methods and the meatballs are tasty either way (and this saves time and extra dishes)!
Give the sauce a taste and season with salt & pepper as needed (I'm generous with both). Serve with cooked spaghetti and some fresh basil (tear it up or slice it thin) and freshly grated parmesan cheese sprinkled over top. Note: Leftovers can be refrigerated/frozen, so if you don't plan on eating it all at once, I would make less than the 1 pound of spaghetti and boil fresh pasta when you're warming up leftover meatballs & sauce.