Castellane with Mascarpone and Roasted Grape Tomatoes

• 2 pints grape tomatoes or cherry tomatoes, halved lengthwise
• 1 lb castellane pasta or medium (regular) shells
• 1 1/4 cups mascarpone cheese (from a 1-lb container)
• 2 oz finely grated Parmigiano-Reggiano (1 cup)
• 1/4 cup minced fresh chives

Preparation

Put oven rack in upper third of oven and preheat oven to 400°F. Line a large shallow baking pan with foil and butter foil.
Arrange tomatoes, cut sides up, in pan and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Roast until slightly plumped, 15 to 20 minutes.
Cook pasta in a 6- to 8-quart pot of boiling salted water , stirring occasionally, until al dente, about 12 minutes. Reserve 1 1/2 cups pasta cooking water, then drain pasta well and transfer to a large bowl. Add mascarpone and stir until melted. Add reserved cooking water, tomatoes, half of parmesan, 3 tablespoons chives, 3/4 teaspoon salt, and 1/2 teaspoon black pepper and toss well, then cool to warm.
Butter a 3-quart (13- by 9-inch) gratin or other shallow baking dish. Toss pasta mixture again, then spoon into gratin dish. Sprinkle remaining parmesan over top. Bake pasta until golden and bubbly, 18 to 20 minutes. Sprinkle with remaining tablespoon chives.