Preheat the oven to 375°F (190°C).
Slice the tops off the tomatoes and gently scoop out the insides using a spoon, being careful not to break the outer walls. Set the hollowed tomatoes aside.
In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped celery, chopped red onion, lemon juice, garlic powder, salt, and pepper. Mix until well combined.
Spoon the tuna mixture into the hollowed tomatoes, packing it in gently.
Place the stuffed tomatoes in a baking dish and sprinkle the shredded cheddar cheese evenly on top of each tomato.
Bake in the preheated oven for 20-25 minutes, or until the tomatoes are tender and the cheese is melted and bubbly.
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley if desired before serving.