In a large skillet, heat the oil over medium heat. Sprinkle the meat with salt and pepper and brown it on all sides. Remove the meat to a large slow cooker.
Add the onion and fennel seeds to the skillet and cook until the onion is tender and golden, about 10 minutes. Stir in the garlic, sage, and rosemary. Add the tomato puree, tomatoes, and a pinch of salt and stir well. Pour the sauce into the slow cooker. Add the crushed red pepper. Cover and cook on low for 5 to 6 hours, or until the pork is very tender.
Remove the meat to a cutting board but leave the sauce in the slow cooker with the heat on. With two forks, tear the pork into bite-size pieces. Return the pork to the cooker to reheat. Stir in the basil.