For the meatballs: In a large bowl, combine the breadcrumbs, Parmesan, basil, parsley, milk, ketchup if using, tomato paste, salt, pepper, eggs, garlic and onions. 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 1 1/2-inch balls (22 to 24 meatballs). Place the meatballs on a heavy nonstick baking sheet, spacing apart. Drizzle with olive oil and bake until cooked through, 15 minutes.
For the tomato sauce: Heat the olive oil in a large deep skillet with 2-inch-high sides over medium-high heat. Add the garlic, onions, salt and pepper. Cook for 5 to 6 minutes. Add 1/2 cup of the basil, the fennel, oregano and crushed tomatoes. Bring to a simmer. Cook over medium heat until the sauce thickens slightly, stirring occasionally, 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.