-
1
For the potatoes: Preheat oven to 400 degrees. Poke sweet potatoes several times with a knife and space them evenly on a foil-lined baking sheet. Bake, turning once, until very tender and easily squeezed with tongs, 1 to 1 1/2 hours. Remove potatoes from oven and cut in half lengthwise to let steam escape; cool at least 10 minutes. Scrape the flesh away from the skins into a large bowl; you should have about 8 cups. (If making ahead, cool to room temperature, and refrigerate.).
-
2
For the streusel: Butter a 9-by-13-inch baking dish. Pulse flour, brown sugar, and salt in a food processor until blended. Sprinkle butter pieces over the flour mixture and pulse until a crumbly mass forms, about 6 to 8 one-second pulses. Sprinkle nuts over the mixture and pulse until combined but some large nut pieces remain, 4 to 6 one-second pulses. Transfer streusel to a medium bowl and return the work bowl to the food processor.
-
3
Place half of the potato flesh into the food processor. With a rubber spatula, break the remaining flesh in the bowl into 1-inch chunks.
-
4
For the filling: Add melted butter, salt, nutmeg, pepper, vanilla, and lemon juice to the potatoes in the food processor. Process until smooth, about 20 seconds. Taste for sweetness, and add up to 4 Tbsp granulated sugar if desired. Add egg yolks and process about 15 seconds. With processor running, pour half-and-half through the feed tube and process until blended, about 30 seconds. Transfer the pureed mixture to the large bowl with the potato chunks. Stir gently until combined.
-
5
Pour filling into the prepared baking dish and spread evenly with a spatula. Sprinkle the streusel over the top, breaking up any large pieces with your fingers. Bake until topping is well browned and filling is puffy around the edges, about 45 minutes. Cool slightly before serving (or cool to room temperature, then cover and refrigerate. To reheat, bake at 400 degrees for about 25 minutes.).
