Grate the medium onion directly into a large mixing bowl to release its juices, which will keep the meatballs moist. Whisk in the egg until well combined, then add the minced garlic, breadcrumbs, grated Parmesan, salt, pepper, and dried oregano. Mix these ingredients together until evenly distributed. Add the ground beef last and fold it in gently with your hands until just combined—overmixing will make the meatballs dense and tough. I prefer to use 80/20 ground beef because the extra fat renders during cooking, keeping the meatballs tender and flavorful rather than dry.

Using damp hands (this prevents sticking), portion the mixture into 24 equal meatballs, roughly the size of a golf ball. Work quickly and gently—overhandling will compress them and result in tough meatballs. Place the formed meatballs on a clean surface or plate as you work. I find that making them slightly undersized allows them to cook more evenly in the slow cooker.

Pour 1½ cups of marinara sauce into the bottom of your slow cooker, creating an even base layer. Arrange all 24 meatballs in a single layer on top of this sauce—avoid stacking them so they cook evenly. Pour the remaining 1½ cups of marinara sauce over the top without stirring or mixing. This layering technique keeps the meatballs from sticking to the bottom and ensures they stay moist and tender throughout cooking.

Cover the slow cooker and cook on high for 3-4 hours or on low for 6-8 hours. The meatballs are done when they're cooked through and have absorbed the flavors of the sauce. The longer, low-temperature cooking method yields more tender meatballs with better flavor development, so choose that option if you have the time.

Transfer the meatballs and sauce to a serving dish or keep warm in the slow cooker on the warm setting. Sprinkle with freshly grated Parmesan cheese and garnish with fresh basil. Serve immediately over pasta, polenta, or with crusty bread for soaking up the sauce.