Mix flour and salt in a food processor. Add the butter in small pieces and mix into a crumbly dough. If the mixture is too dry, add a little teaspoonful of water until the dough forms a ball. Wrap in plastic wrap and place in the refrigerator for 15 minutes.
Preheat oven to 200 degrees C.
Roll out the pastry so that it fits into a 23cm tart tin with a removable bottom and press it up 2cm around the edge of the tin.
Brush pastry lightly with mustard. Sprinkle with cheese and arrange the tomato slices slightly overlapping like roof tiles on top of it. If the tomatoes are very juicy, drain some of the liquid. Bake for 40 minutes in the preheated oven.
Mix parsley, oregano, garlic and olive oil in a small bowl. Remove the tart from the oven and brush with the herb mixture while still hot. Serve warm or reheat in the microwave.