In a medium bowl, combine the drained and flaked tuna, mayonnaise, finely chopped celery, finely chopped red onion, Dijon mustard, and black pepper. Mix well until all ingredients are incorporated.
Spread the tortilla chips evenly in a single layer on a baking sheet.
Spoon the tuna salad mixture evenly over the tortilla chips.
Sprinkle the shredded cheddar and Monterey Jack cheeses evenly over the tuna salad.
Bake in the preheated oven for 8-10 minutes, or until the cheese is melted and bubbly.
Remove from the oven and add any desired optional toppings such as diced tomatoes, sliced jalapeños, chopped cilantro, or a dollop of sour cream.
Serve immediately.
Notes
These tuna melt nachos are best enjoyed fresh from the oven.