1. Heat oven to 400°F. Set 4 heatproof bowls on a jelly-roll pan.
2. Brush tortillas on both sides with oil. Drape over bowls. Bake until tortillas are crisp and hold their shape, 5 to 10 minutes. Allow tortillas to cool completely before removing from cups.
3. Combine tuna, celery, cucumber, mayonnaise, and tomatoes in a bowl. Season with salt and pepper to taste. Place a lettuce leaf in each tortilla bowl. Top with tuna salad.