Wash hands with soap and water.

Preheat oven to 350 °F.

In a medium bowl, stir together brown sugar, flour and oats. Mix in the margarine or butter and set aside.

Spread the rhubarb and blueberries in an 8x8 inch baking dish and set aside.

In a medium saucepan, mix cornstarch and sugar and stir in the juice. Cook over medium heat, stirring all the time, until the mixture has thickened. Turn off the heat.

Stir in vanilla and pour the mixture over the rhubarb and blueberries.

Crumble oat mixture on top of the fruit.

Bake for 45 minutes or until topping is browned and fruit is bubbling at edges. Serve warm or cold.

Try strawberries instead of blueberries.