In a medium mixing bowl, combine the drained tuna and cottage cheese. Stir until well mixed.
Add the diced celery, red onion, and bell pepper to the bowl. Mix until evenly distributed.
In a small bowl, whisk together the mayonnaise, Dijon mustard, lemon juice, salt, and pepper.
Pour the dressing over the tuna mixture and stir gently until everything is well coated.
Taste and adjust seasoning if necessary. If desired, chill in the refrigerator for 30 minutes before serving to enhance the flavors.
Serve on a bed of lettuce, in a sandwich, or with whole-grain crackers. Garnish with fresh parsley if using.