If using imitation crab meat, chop it into bite-sized pieces. For fresh crab meat, gently check for any shell fragments.
In a large mixing bowl, combine the crab meat, celery, and red onion. Add red bell pepper and/or avocado if using.
In a separate small bowl, whisk together the mayonnaise, fresh lemon juice, chopped dill, and Dijon mustard (if using) until smooth.
Pour the dressing over the crab mixture and fold gently with a rubber spatula to coat everything evenly without breaking up the crab meat too much.
Season with salt and freshly ground black pepper to taste.
Let the mixture rest in the refrigerator for at least 10 minutes to allow the flavors to meld together.
When ready to serve, arrange the mixed lettuce leaves on a serving platter or individual plates.
Gently spoon the crab mixture over the lettuce leaves and serve immediately or chill for an additional 15 minutes to enhance flavors.
