In a medium-sized bowl, add drained tuna, Dijon mustard, mayonnaise and lemon juice. Mix everything thoroughly.
Add red onion and finely chopped celery. Sprinkle salt and pepper according to your taste and mix everything until well combined.
Now, warm the skillet over medium heat. Put some olive oil on each side of the bread.
Put the slices of the bread in the skillet and toast until golden brown on each side.
Add a layer of avocado slices on the two pieces of toasted bread, squash them slightly using a fork.
Add a large spoonful of tuna salad on top of avocado.
Layer the tuna salad with tomato slices and arugula or spinach.
Now, top the toasted bread with the remaining slices.
It’s time to serve the tasty tunacado sandwich.