Peel sweet potatoes, halve lengthwise and cut crosswise into 1/4-inch slices. In large pot of boiling water over high heat, parboil sweet potato slices until bright orange and point of sharp knife easily pierces but does not break apart slices, 4 to 5 minutes. Drain well.
Meanwhile, while sweet potatoes are boiling, peel, core and slice apples, then saute in non-stick skillet in 2 tablespoons maple syrup over medium heat until softened but not mushy, 3 to 4 minutes. Remove from heat.
Place sweet potatoes in even layer in buttered 13 x 9 x 2-inch baking dish, overlapping as needed. Top evenly with apples. Sprinkle evenly with pecans, if desired.
In 2-cup measure, whisk together melted butter, remaining 5 tablespoons maple syrup, ginger, cinnamon, nutmeg and salt. In small bowl, whisk together apple juice and cornstarch until smooth. Whisk into butter mixture. Pour evenly over apples and sweet potatoes.
Cover tightly with foil and bake at 375*F (190*C) until liquid is bubbly, about 45 minutes. Uncover and bake until liquid thickens to glaze potatoes and apples, about 15 minutes.