Preheat the oven to 375°F.
In a large bowl, combine the breadcrumbs, Parmesan, basil, and parsley.
In a separate bowl, whisk together the milk, ketchup (if using), tomato paste, salt, and pepper until smooth. Pour this mixture into the breadcrumb mixture and stir until evenly combined.
Add the eggs, garlic, and onion to the mixture and use a fork or your hands to thoroughly combine all of the ingredients.
Add the turkey meat and sausage links to the bowl and mix with your hands until everything is just combined (it should still be slightly lumpy).
Line a baking sheet with parchment paper and lightly oil it.
Using your hands, form the mixture into 1-inch meatballs and place them on the baking sheet (you should have about 30-35 meatballs total). Drizzle a little olive oil over the top of each meatball.
Bake the meatballs for 25-30 minutes, or until they are cooked through and golden brown on top.
While the meatballs are baking, prepare the sauce: heat the olive oil in a large skillet over medium heat. Add the garlic, onion, salt, and pepper and cook, stirring occasionally, until the vegetables are softened and lightly browned, about 8 minutes. Add the basil, fennel seeds, and oregano to the pan and cook for another minute.
Add the crushed tomatoes and chicken broth (if using) to the skillet and bring to a simmer. Lower the heat and simmer for 10-15 minutes, stirring occasionally, until the sauce thickens slightly.
Add the cooked meatballs to the sauce and stir gently to coat them in the sauce. Simmer for an additional 5 minutes, or until heated through. Serve with your favorite pasta and a sprinkle of Parmesan cheese!