Dietary guidance: v
6 ounces Brie, chilled
1 large egg
a thawed 17 1/4-ounce package frozen puff pastry sheets (2 sheets)
3 tablespoons white-wine vinegar
1 teaspoon Dijon mustard
1 teaspoon chutney (preferably tomato)
6 tablespoons extra-virgin olive oil
2 vine-ripened tomatoes
4 cups mesclun (mixed baby greens; about 4 ounces)
Discard rind from Brie and cut into 1/4-inch dice. Freeze Brie cubes, covered, until hard, at least 1 hour, and up to 3 days.
In a bowl lightly beat egg. With a 3 1/2-inch round cutter cut out 4 rounds from 1 pastry sheet and fit rounds into four 1/2-cup muffin cups. Fill cups with frozen Brie cubes. With a 2 1/2-inch round cutter cut out 4 rounds from remaining pastry sheet. Brush 1 side of each round with egg and put 1 round, egg side down, onto each filled cup. With a sharp small knife make steam vents in pastry tops. Chill pastries 30 minutes.
Make dressing while pastries are chilling:
In a small bowl whisk together vinegar, mustard, and chutney and add oil in a stream, whisking until emulsified. Season dressing with salt and pepper. Dressing may be made 1 day ahead and chilled, covered.
Quarter tomatoes and cut out seed sections. Cut quarters into thin strips.
Preheat oven to 425° F.
Bake pastries in middle of oven until deep golden, about 25 minutes. Cool Brie en croûte in cups on a rack 5 minutes and lift from cups, keeping upright. Whisk dressing to combine and in a bowl toss together mesclun, tomatoes, enough dressing to coat, and salt and pepper to taste. Serve Brie en croûte warm with salad.