Frozen Rhubarb Dessert

In a saucepan, bring rhubarb and brown sugar to a boil over medium-high heat, stirring. Reduce heat to medium; cook until rhubarb is softened to sauce consistency, 10-15 minutes.
Meanwhile, grate rind of orange, and squeeze orange to yield 1/4 cup juice.
Mix together juice, rind and cornstarch until smooth; stir into rhubarb sauce and increase heat to high and boil for 1-2 minutes or until thickened slightly.
Transfer to a bowl and let cool to room temperature or chill in refrigerator for about 30 minutes.
METHOD FOR CRUST AND TOPPING:.
Meanwhile, in a food processor, pulse together flour, wallnuts, sugar and cinnamon until crumbly.
Add butter and pulse until combined; press 1 1/2 cups of the crumbs over bottom of 9-inch springform pan.
Bake crust and crumbs in a 350F oven for 7 minutes or until crumbs are golden brown; remove crumbs.
Bake crust for 8 minutes longer or until golden brown. Remove to rack and let cool to room temperature, 25-30 minutes.
In a large bowl, beat egg whites on high speed until soft peaks form; gradually beat in sugar, a spoonful at a time, beating until stiff peaks form. Stir in rhubarb.
Stir in food coloring by drops to desired color. Pour over crust; sprinkle with reserved crumbs. Freeze for 8 hours or over night. (If frozen solid, remove from freezer a few minutes before serving). Run a knife dipped in hot water around the edge and remove springform ring; cut into serving pieces and garnish with mint if desired.