In saucepan, cook rhubarb and lemon juice, stirring occasionally, until fruit is tender (8 to 12 minutes). The rhubarb should release enough moisture to the pan, if needed add 1 to 2 tablespoons water to pot.
Mix sugar, cornstarch and cinnamon. Stir into rhubarb.
Continue cooking, stirring constantly, until mixture boils and thickens.
Remove from heat and set aside.
Crust
Preheat oven to 350°F (180°C).
Combine flours, oats, brown sugar, baking powder, salt and cinnamon.
Add melted butter and mix until no dry spots remain and mixture resembles coarse crumbs.
Press 2/3 or crumbs into bottom of 9x13 inch (23x33 cm) pan.
Spread filling evenly over crust. Sprinkle reserved crumbs on top and press down lightly.
Bake until golden brown, about 35 to 40 minutes.
Remove from oven and set pan on wire rack to cool completely.
Mix icing sugar and just enough milk for the glaze to flow easily.
Drizzle mixture over the cooled bars, before cutting.
Serve warm on a plate with a fork for soft, crumbly bars. For more solid bars you can hold in your hands, let bars set for at least two hours before cutting and serving.