Preheat the oven to 400°. Lightly grease a sheet pan or cover with parchment.
Arrange the tomato slices between layers of paper towels, sprinkle with salt, and leave for 10 minutes to drain.
Carefully unfold the puff pastry on a floured surface. Roll out slightly to flatten. and enlarge the pastry for about an inch in either direction. Transfer to the prepared sheet pan.
With a butter knife score all around the pastry, leaving a 1" border from the edges. Use a fork to prick the pastry every inch or so inside of the border. Don't prick between the border and the edges.
Paint the pastry inside the border with Dijon mustard. Cover the mustard entirely with a layer of sliced Swiss cheese. Arrange the tomato slices over the cheese, overlapping is okay. Drizzle with olive oil and add salt and pepper to taste.
Bake for 15 minutes then turn the oven down to 350°. Bake another 20 minutes until the edges are puffed and browned.
