1. Prepare grill to high heat.
2. Sprinkle steak with salt and black pepper. Combine 1 tablespoon vinegar and 1 tablespoon hoisin in a small bowl; stir with a whisk. Place steak on grill rack coated with cooking spray. Brush steak with hoisin mixture; grill 10 minutes. Turn steak. Brush with hoisin mixture; grill 10 minutes or until desired degree of doneness. Remove from heat; chill 45 minutes. Cut steak into 1/2-inch cubes.
3. Cook noodles according to package directions, omitting salt and fat; drain well.
4. Combine remaining 1 tablespoon vinegar, remaining 11/2 teaspoons hoisin, cilantro, and next 7 ingredients (through garlic) in a large bowl; stir with a whisk. Add beef, noodles, bok choy, and mushrooms; toss to combine. Chill 15 minutes; top with radish, shallots, and mint. Serve with lime wedges and Sriracha, if desired. Yield: 6 servings (serving size: about 11/2 cups).