Do Ahead: Mix together brown sugar, chopped pecans, and cold butter in a small bowl until mixture is grainy and comes together. Cover and chill until ready to use. This can be done up to 3 days in advance.
Wrap each sweet potato in foil and roastat 400F for about 1 hour, or until potatoes are easily pierced by fork and very soft. Actual bake time depends on size of potatoes. Let cool enough to handle. Remove sweet potato skins, and mash potatoes well in a large bowl.
In a separate bowl, beat eggs, maple syrup, vanilla, lemon juice, and salt. Mix in the mashed sweet potatoes until smooth and incorporated. Transfer sweet potato mixture to a buttered 9x13 dish. At this point, you can wrap tightly with cling wrap and put in fridge until ready to use.
Sprinkle pecan topping evenly over casserole. Bake at 350F until casserole is set and topping bubbles. About 1 hour. Serve warm, and enjoy!
Notes
I always choose to roast sweet potatoes in the oven because I really like the deep flavor it creates. You can also cook peeled, cubed potatoes in boiling water until very tender.
Roasted sweet potatoes are typically quite sweet. If your roasted sweet potatoes don't taste very sweet, you can add an additional 1/4 cup of sugar to the sweet potato mixture.
If you want, you can make extra crumble topping, for a thicker layer of crunch on top. Crumble topping can be made up to 3 days in advance and kept in fridge until ready to use.
For substitutions, variation, and serving suggestions, see original article.