3 lb. sweet potatoes
2 tbsp. pure maple syrup
1 tbsp. dark brown sugar
1 tbsp. fresh lemon juice
tsp. ground allspice
3 large egg whites
1/4 tsp. cream of tartar
c. granulated sugar
1) Prepare the potatoes: Preheat oven to 400 degrees F. Pierce sweet potatoes all over with a tip of knife; place in a large microwave-safe bowl. Cover with vented plastic wrap and microwave on High 15 to 17 minutes or until very tender when pierced with fork; drain. When cool enough to handle, peel potatoes and return to the bowl.
2) To bowl with sweet potatoes, add maple syrup, brown sugar, lemon juice, allspice, and 1/4 teaspoon salt. Mash potatoes with potato masher until smooth. Transfer mashed potatoes to a 2-quart casserole dish. (If making ahead, cover and refrigerate up to overnight; bake in 400 degrees F oven for 15 minutes or until heated through.)
3) Prepare meringue: In a large bowl, with a mixer on high speed, beat egg whites and cream of tartar until soft peaks form. Sprinkle in granulated sugar, 2 tablespoons at a time, beating until sugar dissolves and meringue stands in stiff, glossy peaks when beaters are lifted.
4) Transfer meringue to a large piping bag fitted with 1/2-inch plain tip or to heavy-duty gallon-size resealable plastic bag with one corner cut to form 1/2-inch hole. Starting at one side of the casserole dish, pipe meringue in small mounds onto the surface of sweet potatoes, covering the entire surface. Bake for 6 to 8 minutes or until the meringue is golden.