In a large mixing bowl, combine mayonnaise, lemon juice, and Dijon mustard. Mix until smooth.
Break imitation crab into bite-sized chunks and fold gently into the mayo mixture.
Add diced celery, minced onion, and corn. Stir gently to distribute evenly.
Season with salt and pepper. Taste and adjust. Add fresh dill if using.
Cover and refrigerate at least 30 minutes, preferably 1 hour or longer.
Serve cold over lettuce leaves, in sandwiches, or with crackers.