In a food processor or blender, mince yellow onion until puréed. With a cheese grater, grate fresh Mozzarella into fine shreds. Set both aside.
In a large bowl, combine meats, egg, egg white, breadcrumbs, onion, cheese, pesto and seasonings. With wet hands, weave together ingredients until well mixed. Keeping hands damp, roll the mixture into 15 meatballs (about the size of two golf balls).
In a large skillet, heat several tablespoons of extra virgin olive oil over a medium-high flame. Making four at a time, cook meatballs until well browned on all sides, almost to the point of crisp (you will need 10 minutes per batch). When finished, rest on a paper-towel-lined plate to cool.
When all meatballs are cooked in a large pot, heat tomato sauce to a low boil. Add meatballs to the sauce and lower flame, allowing the combination to simmer for 1 – 6 hours. When ready, plate and serve over pasta or with a side of bread.
Notes
If possible, have a splatter screen handy when frying. To know meatballs are finished, always remove one ball from each batch and cut through the middle. When the inside has no pink coloring, they are fully cooked. Wash off skillet in between batches to discard any burnt bits left behind, then re-coat with extra virgin olive oil before frying again.