Preheat the oven to 425F convection. If you don’t have convection, preheat it to 450F. Line a baking sheet with parchment paper.
To make the sauce, in a medium pot, add the tomatoes and break them up using your hands or scissors (be careful of splatter). Add the butter, 3 quarters of the onion (save the remaining ¼ for the meatballs), and a generous pinch of salt (about ½ teaspoon).
Bring to a rapid simmer over medium-high heat, then reduce it to low to maintain a gentle simmer. Cook, uncovered, for 30-40 minutes, or until the sauce has reduced, stirring occasionally. Remove the onion pieces. Taste and season with more salt, if needed. Transfer to a high speed blender and blend until smooth, making sure to keep the lid vented. The sauce will become creamy and delicious. Pour back into the pot.
To make the meatballs, in a medium bowl, add the ricotta, ground oats, parmesan, parsley, egg, garlic, remaining ¼ onion and ½ teaspoon salt and mix together with a spoon. Let sit for 5-10 minutes to hydrate the oats.
Add the meat, remaining ½ teaspoon salt, pepper, and chili flakes (if using). Use your hands to thoroughly mix together. Form into about 10-12 large meatballs (bigger than a golf ball!).
Add 2 tablespoons of oil to the baking sheet. And spread it around. Add the meatballs to the baking sheet and roll each one in the oil to coat it completely. Arrange them evenly on the baking sheet, then bake for 15-20 minutes or until they are deeply browned on the bottoms. Add the meatballs to the pot with the sauce to keep warm.
Meanwhile, cook the pasta (or other starchy base) according to the package instructions. Drain and set aside.
Serve the meatballs over pasta (or other starch). Spoon the sauce over the meatballs and pasta. Top with parmesan, black pepper, and parsley (if using)