1. In a 4- to 6-quart slow cooker, mix together the soy sauce, sugar, vinegar, cornstarch, oil, ginger, and cayenne. Add the ribs and turn to coat.
2. Cover and cook until the ribs are tender, on low for 6 to 7 hours or on high for 3 to 4 hours (this will shorten total recipe time).
3. Twenty minutes before serving, heat broiler. Place the ribs, meaty-side up, on a foil-lined rimmed baking sheet. Strain the cooking liquid from the slow cooker into a medium bowl. Brush the meaty side of the ribs with half of the cooking liquid and broil, basting twice with the pan juices, until shiny and glazed, 3 to 4 minutes.
4. Meanwhile, cook the noodles according to the package directions. Drain and run under cold water to cool. Add the noodles to the bowl of remaining cooking liquid from the slow cooker and toss with the radishes, cilantro, and scallions. Serve the ribs with the udon salad.
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