Cut the tomatoes in slices that are about 1-2 mm thick. Choose a cookie cutter that is slightly larger than the tomato slices.
Cut the puff pastry sheet into as many discs as you can get. Transfer onto a baking pan lined with parchment paper.
Drip a small dollop of mustard on each pastry disc and spread it all over. It doesn't need to be a very thick layer, but of course you can adjust the amount of mustard to your taste.
Place a slice of tomato onto each pastry disc, and add some crumbled feta on top.
Bake in the top section of the preheated oven for 20 minutes at 200°C. I generally switch to broil after the first 5 minutes, so that the tomatoes get properly baked. You can serve these tarts immediately or let them cool to room temperature before serving. You can optionally garnish them with some fresh rosemary.