Drain the tuna and place it in a mixing bowl.

Dice the celery and red onion, and add them to the bowl.

If using, chop the cucumber and halve the cherry tomatoes, then add them as well.

Add mayonnaise, lemon juice, dijon mustard, salt, and pepper to the bowl.

Mix all the ingredients together using a fork until well combined.

Taste and adjust seasoning if necessary.

Serve chilled or at room temperature, optionally garnished with fresh parsley.