In a food processor, grind the onion, fennel, and carrot until fine.
2. In a large saucepan, heat the olive oil over low heat.
Add the ground vegetables, pepper flakes, and garlic and simmer for about 30 minutes, or until the moisture evaporates, stirring occasionally.
3. Brown the chicken and veal, then tomato puree, wine, and stock to the saucepan.
Stir to mix and simmer over medium to high heat, stirring occasionally, for 1½ hours or until the sauce thickens and is reduced by one quarter.
Season to taste with salt and pepper.
4. Bring a large pot of lightly salted water to a boil and cook the pasta for 6 to 8 minutes, until al dente.
5. Drain the pasta and add it to the sauce.
Stir in the butter and sprinkle with Parmesan cheese and serve.