For the bread bag with tuna, first make the bread. For this, mix all dry ingredients. Dissolve yeast in water. Mix all ingredients for the bread and knead for about 10 minutes. Cover and let rest for about 1/2 hour. Shape the dough into a roll.
In the meantime, cut the leek into fine rings. Drain the tuna and mash it in a bowl. Mix with creme fraiche and the leeks. Season with salt and pepper to taste.
Loosely knead the dough and divide into 2 parts. Roll out each part of the dough loosely into a rectangle or flatten by hand to a size of about 25×40 cm.
Divide the mixture between the two dough surfaces and spread so that the edges remain free. Fold in the edges all around. Cut the date tomatoes in half and place on top of the bread pockets. Let rise again for about 1/2 hour.
Heat the oven to 230 °C with a fireproof vessel. Add water to the hot vessel to create enough steam. Reduce the bread pockets and the heat to 200 °C and bake for 35 minutes.
Serve the bread pockets with tuna while still warm.