Combine the leeks, garlic, ginger and jalapeno in a 7-quart slow cooker. Sprinkle in the flour and toss to coat. Nestle the meat in the vegetable mixture. Whisk the hoisin sauce, rice wine, soy sauce, brown sugar and 1/3 cup warm water in a bowl; pour over the meat.
Cover and cook on low, 7 1/2 hours. Remove the meat to a cutting board and thinly slice against the grain; remove and discard the garlic and ginger. Return the meat to the slow cooker along with the bok choy. Increase the heat to high and cook, uncovered, until the bok choy wilts, about 5 minutes. Serve with the rice.