In a medium mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, diced celery, diced red onion, lemon juice, parsley, salt, and black pepper. Stir until thoroughly blended and creamy.

Halve the avocados lengthwise and remove the pits. If needed, scoop out a small amount of flesh to create larger cavities for the filling.

Spoon the tuna salad evenly into the prepared avocado halves.

Drizzle each filled avocado half with olive oil if desired, and sprinkle with paprika or chili flakes. Serve immediately, chilled or at room temperature.