In a bowl, sprinkle pears with lemon juice. In a large mixing bowl, combine 1/2 cup sugar, the 2 tablespoons of flour and lemon peel: stir in sliced pears. Spoon pear-sugar mixture into a 9-inch pastry-lined plate.
In another mixing bowl, combine the 1/2 cup flour, 1/2 cup sugar, ginger, cinnamon and mace. Cut in butter or margarine until mixture resembles coarse crumbs. Sprinkle crumb mixture over pear filling.
To prevent overbrowning, cover edge of pie with foil. Bake at 375 degrees for 25 minutes. Remove foil; bake for 25 to 30 minutes more, or until pie is bubbly and crust is golden. Cool on rack before serving. Garnish with cheese triangles, if desired.