In a saucepan over medium heat, stir together the cornstarch and sugar. Stir in the water. Cook, stirring constantly, until thick and bubbly. Stir in the cherry pie filling and almond extract. Set aside to cool.
Preheat the oven to 350 degrees F (175 degrees C).
In a medium bowl, mix together the flour, oats, brown sugar and salt. Cut in margarine until the mixture is evenly crumbly. Press 2 cups of this mixture into the bottom of a 9x13 inch baking dish. Spread the rhubarb over the crust, then spread the cherry mixture over the rhubarb. Stir the chopped nuts into the remaining crumb mixture, and sprinkle over the top of the fruit.
Bake for 40 minutes in the preheated oven, or until the top is crisp and golden. Serve warm topped with ice cream or whipped topping.