Preheat the oven to 400 degrees F.
In a large bowl, combine the bread crumbs, milk, eggs, onion, Parmesan cheese, basil, parsley, tomato paste, garlic, salt, and pepper. Using a wooden spoon, stir to blend. Add the ground turkey and sausage meat. Using fingers gently mix all the ingredients until thoroughly combined.
Using a 1-ounce cookie scoop or a small ice cream scoop, form the meat mixture into 11/2-inch balls (about 22 to 24 meatballs). Place the meatballs on a heavy non-stick baking sheet, spacing apart. Drizzle with olive oil and bake for 15 minutes until cooked through.
Heat the olive oil in a large deep skillet with 2-inch-high sides over medium-high heat. Add the onions, garlic, salt, and pepper. Cook for 5 to 6 minutes. Add the crushed tomatoes, oregano, fennel, and 1/2 cup basil. Bring to simmer. Cook over medium heat until the sauce thickens slightly, stirring occasionally, about 15 to 20 minutes. Stir in the remaining 1/4 cup basil. Adjust the seasoning with more salt and pepper, if desired.
Add the meatballs to the tomato sauce and simmer until the meatballs are heated through, about 10 minutes. Thin the sauce with chicken stock, 1/4 cup at a time, if needed.
Serve with grated Parmesan cheese alongside.