Sprinkle the ribs with salt and pepper to taste. Roll the ribs in the flour and shake off the excess.
In a large skillet, heat the oil over medium heat and cook th ribs in batches, until browned on all sides. Transfer the ribs to the slow cooker.
Add the onions, carrot, and celery to the skillet and cook, stirring often, until golden, about 15 minutes. Stir in the garlic and herbs and cook for a minute longer. Add the wine and cook, scraping the bottom for the browned bits, until the wine comes to a boil. Stir in the tomato paste, prunes, and broth. Pour over the ribs in the slow cooker.
Cover and cook on low for 8 to 10 hours, or until the ribs are tender and the meat is coming away from the bones. Skim the fat from the surface of the liquid and stir in the mustard. Discard the bay leaves. Taste for seasoning and sprinkle with chopped fresh parsley.