Heat oven to 375. Grease a 9" pie plate. Prepare Streusel Topping; reserve. Arrange rhubarb evenly in plate. Beat remaining ingred. til smooth 15 sec. on high in blender. Pour into plate. Sprinkle evenly with Streusel Topping. Bake until knife inserted in center comes out clean, about 40 minutes. Serve with sweetened whipped cream if desired. STREUSEL TOPPING; Cut butter into baking mix and brown sugar til crumbly; stir in nuts. HIGH ALT> Grease a 10" plate. Increase rhubarb to 2 1/2-3 c. Decrease baking mix to 1/4 c.; add 1/4 c. Flour.