1. Preheat broiler.
2. Cut bell peppers in half lengthwise; discard seeds and membranes. Place pepper halves, skin sides up, on a foil-lined baking sheet; flatten with hand. Broil 20 minutes or until blackened. Place in a zip-top plastic bag; seal. Let stand 20 minutes. Peel peppers, and discard skins.
3. Place 4 bell pepper halves, 3 tablespoons walnuts, breadcrumbs, and next 8 ingredients (through garlic) in a food processor, and process until smooth. Add remaining 2 bell pepper halves; pulse until coarsely chopped. Spoon dip into a bowl; stir in juice. Top with remaining 1 tablespoon walnuts. Serve with radishes, carrots, and lettuce.