In a large Dutch oven or heavy-bottomed pot, heat the olive oil. Add the onion, carrots, celery, fennel seed, crushed red pepper and season with salt and black pepper. Cook over moderate heat, stirring occasionally, until the onion is translucent, about 5 minutes. Add the beef and pork and cook over moderately-high heat, undisturbed, until the meat begins to brown on the bottom, about 5 minutes. Cook, breaking up the meat with your spoon, until it is just cooked through, about 5 minutes more.