Preheat oven to 350° F.
In a medium mixing bowl melt the butter in the microwave. Add flour, oats, Pioneer Golden Light Brown Sugar, cinnamon and allspice.
In a 9-inch square baking pan press half of the mixture into the bottom. Arrange the diced rhubarb and sliced apples evenly on top.
In a saucepan add water, white granulated Pioneer Sugar, cornstarch and vanilla. Cook over medium to medium-high heat until thick. Pour over the rhubarb and apples.
Sprinkle the remaining oat & flour mixture on the top. Bake for 1 hour.
Serve warm with ice cream or whipped cream.