1. Preheat oven to 400 degrees. Pierce sweet potatoes all over with the tip of a knife and place in a large microwave safe bowl. Cover with plastic. Vent with a few small holes. Cook on high power fo 15-17 minutes or until tender.Test by piercing with a fork. Drain. When cool, peel potatoes and return to bowl.
2. Add maple syrup, brown sugar and lemon juice. Mash sweet potatoes with a potato masher until smooth. Transfer to a 2 quart casserole dish.
3. If preparing ahead cover and refrigerate up to overnight. Then bake at 400 degrees for 15 minutes or heated through.

For Meringue

1. In a large mixing bowl with 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.
2. Transfer meringue to large piping bag fitted with 1/2 inch plain tip or to a gallon sized resealable plastic bag with one corner to form 1/2 inch hole. Starting at one side of casserole dish, pipe meringue in small rounds in rows onto the sweet potatoes, covering the top. Bake 6-8 minutes or until meringue is golden.