Arrange sweet potato in a steamer basket over boiling water. Cover and steam 20 minutes or until very tender.
While sweet potato cooks, process cinnamon and cardamom seeds in a coffee grinder or blender until finely ground; set aside, reserving 1/4 teaspoon for top of pie.
When sweet potato is done, cool slightly, and process in a food processor until smooth. Whisk together spices (except for 1/4 teaspoon), sweet potato, eggs, and next 5 ingredients in a large bowl.
Combine flour and 1/2 teaspoon salt. Cut in shortening with a pastry blender until the size of small peas. Sprinkle ice water, 1 tablespoon at a time, evenly over surface; stir with a fork until dry ingredients are moistened. Shape dough into a ball. Roll dough to about 1/4" thickness on a lightly floured surface. Fit into an ungreased 9 1/2" deep-dish fluted tart pan; trim off excess pastry along edges. Line tart shell with aluminum foil, pressing foil into the flutes. Trim foil to within 1/2" of top of pan. Fold foil down over top edge of crust to prevent overbrowning.
Bake at 425º for 15 minutes; remove foil, and bake 7 minutes or until pastry is golden. Reduce oven temperature to 350º. With tart pan still on oven rack, pull out rack; pour filling into pastry, and sprinkle with remaining 1/4 teaspoon spices. Bake at 350º for 1 hour or until set. Cool completely before serving. Garnish, if desired. Store in refrigerator.
Note: If you don't have a fluted tart pan, you can bake this pie in 2 (9") glass pieplates. Fit each piecrust (from a 15-ounce package refrigerated piecrusts) into a 9" pieplate according to package directions; fold edges under, and crimp. (No need to prebake crusts for these smaller pies.) Pour filling evenly into 2 prepared piecrusts. Sprinkle with remaining 1/4 teaspoon spices. Bake at 450º for 15 minutes. Reduce heat to 350º; bake 30 more minutes or until set. Cool on a wire rack. (Pies will be thin.) Or bake pies using 2 frozen piecrusts.
Tip: Use a meat mallet to break cinnamon stick before placing in coffee grinder. Tap cardamom pods lightly with mallet to release seeds.