Place red peppers, onions and lemon juice in the bowl of a food processor or blender; cover and purée until smooth. Stir in sour cream, green pepper, red pepper flakes and salt.
Transfer dip to a serving bowl; cover and refrigerate for 2 to 3 hours to allow flavors to blend. Serve with celery and carrot sticks, sliced mushrooms, cauliflower and radishes. Yields 3 cups.