How to Make It
Preheat oven to 425°F. On a floured piece of parchment, roll puff pastry into a 10-by-12-inch rectangle; prick all over with a fork. Transfer parchment and pastry to a baking sheet.
Whisk sour cream, mustard, chopped thyme, ¼￼ teaspoon salt, and several grinds of pepper in a small bowl. Spread sour cream mixture evenly over pastry, leaving a ￼½-inch border. Top with tomatoes, overlapping slightly, and season with remaining ½￼ teaspoon salt and several grinds of pepper.
Bake until pastry is golden brown (peek underneath to check the bottom) and tomatoes look dry, 30 to 35 minutes. Let cool slightly before cutting into squares. Serve topped with thyme leaves and flaky salt.