Place tomato slices on a triple layer of paper towels. Sprinkle with salt and let sit for 30 minutes.

Preheat oven to 400F.

On a lightly floured surface, roll out pastry into a large rectangle, about 9-by-11-inch, and place on a baking sheet lined with parchment paper. Fold over the edge and pleat to form a ½-inch border. Poke the pastry all over with a fork.

Spread Dijon mustard over the pastry evenly with a pastry brush. Arrange sliced brie and tomatoes over the pastry, overlapping slightly. Sprinkle evenly with dried thyme.

Bake until pastry is puffed up at the edges and golden brown, 20-25 minutes. Cut into wedges or squares and serve immediately.