Begin by draining the canned tuna thoroughly to avoid a soggy sandwich.

Wash and slice fresh cucumbers into thin discs.

In a mixing bowl, combine drained tuna, mayonnaise, and mustard.

Add diced celery and finely chopped red onion.

Season with salt and pepper to taste.

Gently fold in sliced cucumbers until evenly distributed.

Decide whether to toast bread; if so, preheat toaster or oven to 350°F (175°C) and toast for 5 minutes.

Spread a generous amount of the tuna salad onto a slice of bread and top with another slice.

Cut sandwich diagonally for serving.

Serve with sides like vegetable sticks or chips.