Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the drained tuna, mayonnaise, cheddar cheese, celery, red onion, Dijon mustard, garlic powder, salt, and pepper. Mix well until all ingredients are fully incorporated.
Scoop out a little of the flesh from each avocado half to create a small bowl for the tuna mixture.
Drizzle olive oil over the avocado halves and season with a pinch of salt and pepper.
Fill each avocado half with the tuna mixture, packing it in gently.
Place the filled avocado halves on a baking sheet and bake in the preheated oven for 10-12 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool slightly. Garnish with chopped parsley if desired.