First, drain the can of tuna well. Cut the avocado in half lengthwise. Twist the halves apart and gently remove the pit with a spoon. Leave the avocado skin on, as it forms the base for the dish.
Place the drained tuna in a bowl and flake the fish with a fork into smaller pieces. Add the tablespoon of sugar-free mayonnaise to this. Mix everything thoroughly until a cohesive tuna salad forms. Season the mixture generously with pepper.
Take the hollowed avocado halves and place them stably on the work surface. Spoon the tuna salad into the hollows where the pit was. Distribute the salad evenly over the two halves and press gently.
Place the filled avocado halves on a plate. Serve the dish immediately so the avocado stays beautifully green and fresh.