Preheat oven to 425°F.
For the sauce, heat a Dutch oven or heavy-bottomed pot over medium heat with EVOO. Add fennel, onions, garlic and salt, and cook to soften, 10 minutes. Add stock or water and let it absorb, another few minutes, then add tomatoes and simmer 20 minutes. Purée with an immersion blender or in a food processor then return to pot. Keep over a low simmer, add torn basil and stir in aged balsamic to finish sauce.
Place meats in a bowl and season with salt, both peppers and fennel seeds. Add breadcrumbs, milk, egg, cheese, parsley and EVOO and mix well with your hands. Roll into golf-size meatballs and place on a parchment-lined baking sheet. Transfer baking sheet to oven and partially cook the meatballs 10-12 minutes until light golden brown but not firm. Transfer meatballs to sauce and let them simmer with sauce for 15 minutes to finish cooking through. Transfer meatballs to a bowl and cover to keep warm.
Meanwhile, bring a large pot of water to a boil. Add a generous amount of salt and cook pasta 1 minute shy of recommended cooking time. Reserve 1 cup starchy cooking water, drain pasta and add it back to the hot pot along with the butter, cheese and half the sauce. Stir well, using the starchy water to emulsify the sauce and coat the pasta.
Serve pasta in shallow bowls topped with more sauce and some meatballs. Pass cheese at the table.