Because natural sugar levels in sweet potatoes vary greatly depending on variety, size, and season, it's important to taste the filling before adding sugar. If the filling is bland, add up to 4 tablespoons sugar; if the potatoes are naturally sweet, you may opt to omit the sugar altogether. When sweetening the filling, keep in mind that the streusel topping is quite sweet. If you can find them, Beauregard, Garnet, or Jewel sweet potatoes have the best texture for this recipe.

For even cooking, buy potatoes that are uniform in size. Avoid potatoes larger than 1 1/2 pounds; they require a longer roasting time and tend to cook unevenly. The potatoes can be baked up to 2 days ahead. Scrape the flesh from the skins and refrigerate in an airtight container. To serve 4 to 6, halve all the ingredients and bake the casserole in an 8-inch-square baking dish for 35 to 40 minutes.

Directions
1. For the sweet potatoes: Adjust oven rack to lower-middle position and heat oven to 400 degrees. Poke sweet potatoes several times with paring knife and space evenly on rimmed baking sheet lined with aluminum foil. Bake potatoes, turning them once, until they are very tender and can be squeezed easily with tongs, 1 to 1 1/2 hours (or 45 minutes for small sweet potatoes). Remove potatoes from oven and cut in half lengthwise to let steam escape; cool at least 10 minutes. Reduce oven temperature to 375 degrees.

2. For the streusel: While potatoes are baking, butter 13 by 9-inch baking dish. Pulse flour, brown sugar, and salt in food processor until blended, about four 1-second pulses. Sprinkle butter pieces over flour mixture and pulse until crumbly mass forms, six to eight 1-second pulses. Sprinkle nuts over mixture and pulse until combined but some large nut pieces remain, four to six 1-second pulses. Transfer streusel to medium bowl and return empty workbowl to processor.

3. Once potatoes have cooled slightly, use spoon to scoop flesh into large bowl; you should have about 8 cups. Transfer half of potato flesh to food processor. Using rubber spatula, break remaining potato flesh in bowl into coarse 1-inch chunks.

4. For the filling: Add melted butter, salt, nutmeg, pepper, vanilla, and lemon juice to potatoes in food processor and process until smooth, about 20 seconds. Taste for sweetness, then add up to 4 tablespoons granulated sugar, if necessary; add yolks. With processor running, pour half-and-half through feed tube and process until blended, about 20 seconds; transfer to bowl with potato pieces and stir gently until combined.

5. To assemble and bake casserole: Pour filling into prepared baking dish and spread into even layer with spatula. Sprinkle with streusel, breaking up any large pieces with fingers. Bake until topping is well browned and filling is slightly puffy around edges, 40 to 45 minutes. Cool at least 10 minutes before serving.