Line a large baking sheet with parchment paper and set aside.
In a large bowl, add the onion, garlic, breadcrumbs, parsley, salt, pepper, parmesan, milk, Calabrian chiles, and egg. Mix well to combine. Add the beef and pork and mix until everything is fully incorporated.
Roll into 1 ½-inch (3.8 cm) meatballs and place on the baking sheet.
Heat the olive oil in a deep sauté pan or dutch oven set over medium-high heat. Place the meatballs in the pan and brown for 3-4 minutes on each side, reducing the heat slightly if needed. Once browned, set aside on a plate.
For the sauce, reduce to medium heat and add the 2 tbsp of white wine to deglaze the pan, scraping up any brown bits. Add the onions and sauté until softened and translucent.
Add the tomatoes, breaking them up with a wooden spoon. Tear up the basil leaves and add them to the sauce along with the salt. Stir to combine everything then bring to a boil.
Once boiling, reduce to low heat and nestle the meatballs into the sauce. Simmer for 25 minutes or until the internal temperature reaches 160°F (71°C). Serve warm.