• 12 ounces mixed heirloom tomatoes (about 3 medium), sliced ¼ in. thick
• 1 cup cherry tomatoes, halved
• 1¼ teaspoon kosher salt, divided
• 3 ounces sharp Cheddar cheese, shredded (about 1 cup)
• ½ cup mayonnaise
• 2 scallions, finely chopped
• 1 tablespoon chopped fresh dill
• Pinch of cayenne
• ½ teaspoon freshly ground black pepper, divided
• ¼ cup olive oil
• frozen puff pastry, thawed & placed end to end on baking sheet
Preheat oven to 450°F with rack in lowest position. Arrange tomatoes on a large baking sheet lined with paper towels. Season with 1 teaspoon salt and let stand 10 minutes. Pat dry with paper towels.
Stir together cheese, mayonnaise, scallions, chopped dill, cayenne, and ¼ teaspoon pepper in a medium bowl. Place oil in a small bowl. Line a baking sheet with parchment paper, brush with oil, and place pastry on top; brush with oil. Spread cheese mixture evenly over top, leaving a ½-inch border. Top with tomatoes and season with remaining ¼ teaspoon each salt and pepper.
Bake until crust is golden brown, 18 to 20 minutes; let cool 5 minutes. Top with dill sprigs just before serving.