In this hearty variation on traditional focaccia, mashed fingerling potatoes are incorporated into the dough, which is topped with thin slices for baking. Fingerlings are grown in yellow, pink, and blue varieties. For a special touch, use an assortment of them. Any small potatoes will work just as well as fingerlings.

7 tablespoons olive oil, plus more for bowl and plastic wrap

1 pound assorted fingerling potatoes

4 1/2 teaspoons coarse salt, plus more for seasoning

1 package active dry yeast

6 cups all-purpose flour

1 1/2 tablespoons fresh rosemary, coarsely chopped

Freshly ground pepper

Pour 2 tablespoons olive oil into a 12-by-18-inch rimmed baking pan, and spread all over using fingertips; set aside. Place half the potatoes in a small saucepan. Cover with cold water; place over high heat. Add 3 teaspoons salt when water comes to a boil. Reduce heat to medium-high; cook until potatoes are tender, 10 to 12 minutes. Drain in a colander, reserving liquid. Pass through a potato ricer or a food mill and into a bowl; set aside.
Place 1/4 cup reserved warm liquid into the bowl of an electric mixer fitted with the paddle attachment. Sprinkle in yeast, and stir well; let stand until creamy, 5 to 10 minutes. Add 2 1/4 cups more reserved liquid, 2 tablespoons olive oil, and the reserved mashed potatoes; beat until combined. In a large bowl, whisk together flour and remaining 1 1/2 teaspoons salt; add to potato mixture. Mix on low speed until flour is incorporated, about 3 minutes. Change to the dough hook, and knead on medium-high until dough is smooth and elastic and is slightly tacky when squeezed but does not stick to fingers, 4 to 5 minutes.
Turn dough out onto a clean surface, and knead into a ball. Place in a lightly oiled large bowl, cover with plastic wrap, and let stand at room temperature until doubled in size, 1 to 1 1/2 hours.
Preheat oven to 425 degrees. Spread dough evenly in the prepared baking pan. Cover with oiled plastic wrap, and let stand in a warm place until dough has filled the entire pan and has increased in size by about one-third, about 30 minutes.
Using a mandoline or a knife, slice remaining potatoes into very thin rounds. Transfer to a bowl. Add half the rosemary and 1 tablespoon olive oil; season with salt and pepper. Toss to coat potatoes well.
Remove plastic wrap; dimple dough with fingertips, leaving deep indentations. Drizzle with remaining 2 tablespoons oil. Gently press reserved sliced potatoes into dough. Sprinkle with remaining rosemary. Bake until golden brown, 30 to 35 minutes. Remove from oven; transfer to a wire rack. Serve warm.