Preheat your oven to 400°F and line a baking sheet with parchment paper.
In a large bowl, combine ground beef, Italian breadcrumbs, milk, eggs, diced onion, salt, black pepper, oregano, minced garlic, parmesan cheese, and fresh parsley.
Mix gently until everything is combined. Be careful not to overmix, as this can make the meatballs tough.
Using a large cookie scoop, portion the mixture into evenly sized meatballs.
Wet your hands slightly with water and roll the meatballs until smooth, about 2 inches in diameter. This recipe makes about 19 meatballs.
Place the meatballs on the prepared baking sheet.
Bake for 18–20 minutes, or until cooked through and browned.
While the meatballs are baking, bring a large pot of salted water to a boil.
Cook the spaghetti noodles according to package directions, about 10 minutes, until al dente.
Drain and set aside.
In a large skillet or saucepan, heat the olive oil over medium heat.
Add the diced onion and cook until softened and translucent.
Stir in the garlic and cook for about 1 minute, until fragrant.
Pour in the marina sauce and bring to a gentle simmer.
Allow the sauce to simmer for about 10 minutes to develop flavor.
Add the meatballs to the sauce and toss until evenly coated.
Add the cooked spaghetti noodles to serving plates, and top with the sauce and meatballs.
Finish with fresh basil and freshly grated Parmesan cheese before serving.