- 3 pounds yellow-fleshed potatoes such as Yukon Gold
- 1/2 cup milk
- 1 stick (1/2 cup) unsalted butter
- 1 cup Kalamata olives
- 4 scallions
- 1 garlic clove
- 1/2 cup fresh parsley leaves
- 3 cups fresh bread crumbs
- 1/2 cup freshly grated Parmesan (about 1 1/2 ounces)
- 3 pounds vine-ripened red or yellow tomatoes (about 7 medium)
- In a 4-quart heavy kettle cover potatoes with salted cold water by 2 inches and simmer until tender, about 30 minutes. While potatoes are simmering, in a small saucepan heat milk with 6 tablespoons butter over moderate heat until butter is melted. Remove pan from heat and keep mixture warm, covered. Pit olives and separately chop olives and scallions. In a colander drain potatoes and when just cool enough to handle, peel. Coarsely smash potatoes with a potato masher and gently fold in milk mixture, olives, scallions, and salt and pepper to taste.
- Preheat oven to 400°F. and butter a 3-quart gratin dish (about 15 by 10 by 2 inches).
- Mince garlic and chop parsley. In a nonstick skillet heat remaining 2 tablespoons butter over moderate heat until foam subsides and cook garlic, stirring, until fragrant, about 1 minute. Add bread crumbs and cook, stirring, until golden. Transfer mixture to a bowl and cool. Stir in parsley and Parmesan and season with salt and pepper.
- Cut tomatoes into 1/4-inch-thick slices. Spread potatoes in gratin dish and top with tomatoes, overlapping them to completely cover potatoes. Sprinkle crumb mixture over tomatoes and bake gratin in middle of oven until top is golden brown and tomatoes are tender, about 25 minutes. Gratin may be made 1 day ahead and cooled completely, uncovered, before being chilled, covered. Reheat gratin, uncovered.