Thaw frozen rhubarb completely, if using; drain well and discard liquid. In a large bowl combine fresh or thawed rhubarb, pineapple, and the 1 cup brown sugar. Cover and let stand 1 hour. Drain mixture, reserving juices. If necessary, add water to reserved juices to equal 2/3 cup.
In a small saucepan, stir reserved juices into cornstarch. Cook and stir over medium heat until thickened and bubbly. Remove from heat. Stir into rhubarb mixture; stir in lemon peel. Place mixture in a 2-quart square baking dish.
In another bowl combine flour, the 1/4 cup brown sugar, granulated sugar, ginger, and salt. Cut in butter with a pastry blender until mixture is crumbly. Spoon over top of fruit.
Bake in a 350 degree F oven about 1 hour or until light brown and bubbly. Serve warm with ice cream, if desired. Makes 6 to 8 servings.