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. Break remaining potato flesh in bowl into coarse 1-inch chunks.
Add melted butter, salt, nutmeg, pepper, vanilla, and lemon juice to potatoes in food processor and process until smooth about 20 sec.
Taste for sweetness, then add up to 4 Tbsp sugar, if necessary.
Add yolks. With processor running, pour half-and-half through feed tube and processs until blended, about 20 sec.
Transfer to bowl with potato pieces and stir gently until combined.
Pour filling into prepared baking dish and spread into even layer. 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 min. Cool at least 10 min prior to serving.