Make Topping Ahead: Mix together brown sugar, chopped pecans, cold butter, and cinnamon in a small bowl until mixture is grainy, damp, and comes together. Cover and chill until ready to use. Can be made 1-2 days ahead.
Preheat oven to 400F with rack on lower middle position. Wrap each sweet potato in foil and bake about 1 hour, or until easily pierced by a fork and very soft. Let cool enough to handle. Remove sweet potato skins, place potatoes in a large bowl and mash them well.
In a separate bowl, beat eggs, maple syrup, vanilla, lemon juice, and salt. Mix in the mashed sweet potatoes until smooth and incorporated. Transfer mixture into a buttered 9x13 casserole dish.
Sprinkle pecan topping evenly over casserole. Bake uncovered at 350F until casserole is set and topping bubbles, about 1 hour.
Notes
I always choose to roast whole sweet potatoes with skin on in the oven, as it creates deep roasted flavors. You can also cook peeled, cubed potatoes in boiling water until very tender.
You can roast potatoes a day ahead. Actual cook time depends on size of sweet potatoes.
Typically, roasted sweet potatoes lend a good amount of natural sweetness to the dish. If your roasted sweet potatoes don't taste very sweet, you can add 1/4 cup extra sugar to the mixture.
If desired, make extra crumble topping for a thicker layer of crunchy streusel on top. Topping can be made up to 3 days in advance; keep airtight in fridge until ready to use.
My favorite way to use leftovers of this casserole is in this sweet potato bread!
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