Preheat the oven to 450 degrees F. Set the ribs in a single layer, bone side down, in a roasting pan; season with salt and pepper. Roast the ribs for 30 minutes, or until richly browned. Set the ribs in your slow cooker, bone side down.
Place the oil in a skillet set over medium-high heat. Add the mushrooms and onion and cook until tender, about 5 minutes. Mix in the flour and garlic and cook for 2 minutes. Slowly mix in the stock and wine. Add the thyme, bring the sauce to a simmer, and then pour it over the ribs. Cover and cook on the low setting for 6 to 8 hours, or until the ribs are tender. Skim off any fat from the sauce, then season to taste with salt and pepper, if needed. Top the ribs with the sauce and sprinkle with parsley to serve.