8 stalks rhubarb, cut into 1/2-inch pieces
3 apples - peeled, cored, and diced
½ cup white sugar
¼ cup water
Cake:
⅔ cup white sugar
¼ cup margarine
1 egg
1 teaspoon vanilla extract
¾ cup milk
1 ½ cups all-purpose flour
2 teaspoons baking powder
¼ teaspoon salt
Crumb Topping:
1 ½ cups rolled oats
½ cup margarine
½ cup white sugar
1 teaspoon ground cinnamon
Combine rhubarb, apples, 1/2 cup white sugar, and water together in a saucepan; bring to a boil. Cook and stir rhubarb mixture until rhubarb is tender and compote is thickened, about 10 minutes. Preheat oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish. Mix 2/3 cup white sugar, 1/4 cup margarine, egg, and vanilla extract together in a bowl. Stir milk and stir into the sugar mixture. Whisk flour, baking powder, and salt together in a separate bowl. Mix flour mixture into wet mixture until batter is just moistened; pour into the prepared baking dish. Spoon enough compote over cake to make about 1/2-inch-thick layer, reserving any leftover compote for another use. Stir oats, 1/2 cup margarine, 1/2 cup white sugar, and cinnamon together in a bowl until crumbly; sprinkle over compote layer. Bake in the preheated oven until a toothpick inserted in the middle of the cake comes out clean, 30 to 40 minutes.