Preheat the oven to 190 °C top/bottom heat (convection oven 170 °C). Carefully pull the puff pastry straight from the fridge apart so that it becomes slightly larger.
Grease the tart mould and put the dough into it. Form a nice edge with the protruding dough.
Spread the mustard on the dough base. Remove the stems from the ox-heart tomatoes, cut them into slices, place them in a circle in the tin and pepper well. Grate the Gruyère and sprinkle the tomatoes well.
Bake the tart in the oven for about 35 minutes. Rinse the spring leek, dab dry, cut into small rolls, sprinkle over the baked tart and serve hot immediately. A green salad goes well with it.