Make crust: Combine flour, sugar and salt in processor. Using on/off turns, cut in shortening and butter until coarse meal forms. Blend in enough ice water 2 tablespoons at a time to form moist clumps. Gather dough into ball; cut in half. Flatten each half into disk. Wrap separately in plastic; refrigerate until firm, about 1 hour. (Can be made 1 day ahead. Keep chilled. Let dough soften slightly at room temperature before rolling.)
Make filling: Preheat oven to 400 degrees F. Combine first 7 ingredients in large bowl. Toss gently to blend. Roll out 1 dough disk on floured work surface to 13-inch round. Transfer to 9-inch-diameter flass pie dish. Trim excess dough, leaving 3/4-inch overhang. Roll out second dough disk on lightly floured surface to 13-inch round. Cut into fourteen 1/2-inchwide strips.
Spoon filling into crust: Arrange 7 dough strips atop filling, spacing evenly. Form lattice by placing remaining dough strips in opposite direction atop filling. Trim ends of dough strips even with overhang of bottom crust. Fold strip ends and overhang under, pressing to seal.
Crimp edges decoratively. Brush glaze over crust. transfer pie to baking sheet. Bake 20 minutes. Reduce oven temperature to 350 degrees F. Bake pie until golden and filling thickens, about 1 hour 25 minutes. Transfer pie to rack and cool completely.
Make topping: Combine mascarpone with scraped vanilla bean and powdered sugar.
To serve: Cut pie into 8 slices and serve with dollop of mascarpone.