Drain the Tuna: Empty both cans of tuna into a fine-mesh strainer and press gently with the back of a spoon to remove excess liquid, about 1 minute.
Mix the Base: In a medium bowl, combine the drained tuna, diced celery, mayonnaise, garlic powder, and black pepper, stirring until evenly combined.
Taste and Adjust: Add a squeeze of fresh lemon juice or extra mayonnaise if desired, then season to your preference, about 30 seconds.
Halve and Seed: Slice each bell pepper in half lengthwise, then carefully scoop out the seeds and white membranes with a small spoon, leaving the pepper wall intact.
Pat Dry: Use a paper towel to gently dry the inside of each pepper half, removing any excess moisture so the filling stays anchored.
Arrange on Pan: Place the pepper halves cut-side up on a baking sheet or shallow baking dish, spacing them about 1 inch apart.
Fill the Peppers: Spoon the tuna mixture evenly into each pepper half, pressing gently so it fills the cavity and creates a small mound on top.
Top with Cheese: Place one slice of cheese over each filled pepper half, covering the tuna mixture generously.
Bake: Place in a preheated 400°F oven for 12-15 minutes, until the cheese is melted and lightly golden and the peppers are tender-crisp.
Finish and Serve: Remove from the oven and let cool for 1-2 minutes before serving, optionally garnishing with fresh herbs or additional pepper flakes.