In a bowl, combine the flour, oil, water, and a pinch of salt. Knead quickly until you get a smooth ball of dough. This only takes a few minutes.
Roll out the dough and place it in a pie pan lined with parchment paper. Poke small holes in the bottom of the pie with a fork.
Preheat the oven to 180°C (350°F).
Spread the mustard over the bottom of the pie.Sprinkle with semolina; this helps absorb the water from the tomatoes. Drain and flake the tuna, then sprinkle with mustard.
Slice the tomatoes and sprinkle them over the tuna.
Cover with grated cheese and bake on the bottom rack for at least 30 to 40 minutes, until the cheese is melted.
Enjoy your meal!