PreparationFor streusel: Place all ingredients in processor in order listed. Using on/off turns, blend until small moist clumps form. Transfer to bowl. Cover and refrigerate at least 1 hour. (Can be made 2 days ahead. Keep refrigerated.) For sweet-potato puree: Preheat oven to 375°F. Pierce potatoes in several places with fork. Place directly on oven rack and bake until very tender, about 1 hour. Transfer to rack and cool. Peel potatoes; place flesh in large bowl and mash. Mix in orange juice, sugar, butter, and orange peel. Season with salt and pepper. Butter 13x9-inch oval gratin dish. Spread puree in dish; smooth top. (Puree can be made 1 day ahead. Cover and chill.) Preheat oven to 400°F. Scatter streusel over potatoes. Bake until puree is hot and streusel is crisp, about 40 minutes. Cool 5 minutes, then serve.
PreparationFor streusel: Place all ingredients in processor in order listed. Using on/off turns, blend until small moist clumps form. Transfer to bowl. Cover and refrigerate at least 1 hour. (Can be made 2 days ahead. Keep refrigerated.) For sweet-potato puree: Preheat oven to 375°F. Pierce potatoes in several places with fork. Place directly on oven rack and bake until very tender, about 1 hour. Transfer to rack and cool. Peel potatoes; place flesh in large bowl and mash. Mix in orange juice, sugar, butter, and orange peel. Season with salt and pepper. Butter 13x9-inch oval gratin dish. Spread puree in dish; smooth top. (Puree can be made 1 day ahead. Cover and chill.) Preheat oven to 400°F. Scatter streusel over potatoes. Bake until puree is hot and streusel is crisp, about 40 minutes. Cool 5 minutes, then serve.