In a blender, puree the mayonnaise, roasted pepper, 1 tablespoon of the lemon juice, and the cayenne pepper. Set aside. In a medium bowl, toss together the crabmeat, avocado, onion, remaining lemon juice, salt, and pepper.
Cut the top third off pita breads. Divide crabmeat mixture among pitas, drizzle with red pepper mayonnaise, and tuck some alfalfa sprouts into each sandwich. Tightly wrap in waxed paper and keep chilled up to 1 hour.