In a small saucepan, whisk sugar and cornstarch together. Add blueberries and strawberries, then boil down over medium heat.
Once the berries have simmered down and are soft, add apple and raisins. Gently simmer until apples are slightly softened. Remove from heat and stir in vanilla and immediately pour fruit filling into a small skillet.
Preheat oven to 350 degrees F.
In a small skillet, melt butter. Add sugar, oats, and flour. Over medium-low heat, toss the oats until they become slightly toasted. Remove from heat.
Sprinkle the rolled oats over the fruit filling.
Bake for about 10-15 minutes or until the oats are golden and the fruit is bubbling around the sides.