To make apples: Leave peels on apples. Remove cores with a small apple corer or melon baller. Cut apples into 1/2-inch-thick rings. Fill a nonreactive medium bowl with 2 cups water and squeeze lemon into it. Place apple slices into acidified water while you make marinade.
To make marinade: Combine maple syrup, ginger root, lemon juice, orange juice and spice powder in a nonreactive medium bowl. Drain apples and place in marinade 20 minutes while you heat grill. Grill apple slices over medium-high heat 2 to 3 minutes per side until golden while basting with marinade. Pour any remaining marinade over apples and serve sprinkled with chopped nuts.