Wash the celery, peel off the threads and cut the celery into small pieces. Simmer for a few minutes in a non-stick pan with a little water. Add the tuna with juice and mix well. Steam over medium heat until the celery is tender. Let cool in a bowl.
Preheat the oven to 180 degrees.
Roll out the shortcrust pastry and place in a baking pan.
Beat the eggs, mix with the pecorino into the tuna and celery mixture, stir in the sour cream and season with salt and pepper. Spread on the shortcrust pastry and cover with cheese slices of your choice.
Bake the tart for 30-35 minutes. Let it cool down a bit and serve it lukewarm.