Place the slices of tomatoes on paper towels to drain. Put a circle of puff pastry or pastry crust on a sheet pan. Add a thin layer of Dijon mustard to the pastry, leaving an inch border around the edge. Arrange the slices of tomatoes on top of the mustard. Overlapping tomatoes are fine. Lightly add salt and pepper to the tomatoes. If using, add aged goat cheese slices on top of the tomatoes. Fold over the edges of the crust. Bake according to the time indicated for the pastry, about 30 minutes or until the tart is golden brown and bubbly.