In a mixing bowl, combine 200 grams of canned tuna (drained and flaked), 1/4 cup of finely chopped onion, 1/4 cup of finely chopped celery, 1/4 cup of finely chopped capsicum, and 2 tablespoons of chopped coriander leaves.
Add 1/4 cup of mayonnaise, 1 tablespoon of lemon juice, salt, and black pepper to taste. Mix well until all ingredients are evenly combined.
Cut a 60 cm long baguette into two equal halves and then slice each half lengthwise, without cutting all the way through, to create a pocket.
Spread a thin layer of butter on the inside of each baguette half.
Fill the baguette pockets with the prepared tuna salad mixture, distributing it evenly.
Add lettuce leaves and tomato slices on top of the tuna salad, if desired.
Close the baguette halves and press gently to secure the filling. Slice each half into 2 or 3 pieces and serve immediately.