Step 1

Preheat oven to 400°F, with a rack in center. On a lightly floured sheet of parchment, roll pastry out to a 9 1/2-by-11 1/2-inch rectangle (1/8 inch thick). Trim edges straight. Refrigerate on a baking sheet 10 minutes.

Step 2

Sprinkle tomatoes with salt; arrange on a layer of paper towels. Top with more paper towels (to absorb excess liquid).

Step 3

Score edges of pastry, creating a 1/2-inch border. Spread mustard inside border, then top with cheese, tomatoes, capers, and thyme, overlapping tomatoes slightly. Drizzle with oil, season with salt and pepper, and bake 15 minutes.

Step 4

Reduce temperature to 375°; continue baking until golden brown and cooked through, 25 to 35 minutes more. Sprinkle with thyme sprigs and serve warm or room temperature, with greens.