Preheat your oven to 400°F (200°C).

In a large mixing bowl, combine breadcrumbs, Parmesan cheese, basil, parsley, milk, ketchup (if using), tomato paste, salt, pepper, eggs, garlic, and onions. Stir with a wooden spoon until the ingredients are well-mixed.

Add the ground turkey and sausage meat to the bowl. Using your hands, gently combine all the ingredients until evenly distributed.

Using a 1-ounce cookie scoop or small ice cream scoop, shape the mixture into 1 1/2-inch balls (you should get 22 to 24 meatballs). Arrange the meatballs on a nonstick baking sheet, making sure they are spaced out.

Drizzle with olive oil and bake for about 15 minutes, or until fully cooked through.

For the Tomato Sauce:

Heat olive oil in a large, deep skillet (with sides about 2 inches high) over medium-high heat.

Add garlic, onions, salt, and pepper to the skillet and sauté for 5-6 minutes.

Stir in 1/2 cup of basil, fennel, oregano, and crushed tomatoes. Bring to a simmer and cook, stirring occasionally, for 15-20 minutes, or until the sauce thickens slightly.

Stir in the remaining 1/4 cup basil and adjust the seasoning with additional salt and pepper, if desired.

Final Steps:

Gently add the baked meatballs to the tomato sauce, and simmer for an additional 10 minutes until the meatballs are fully heated through.

If the sauce is too thick, thin it out by adding chicken stock, a little at a time, until the desired consistency is reached.

Serve the meatballs and sauce with freshly grated Parmesan on the side.