½ cup strawberries, hulled and quartered (fresh or frozen)
½ teaspoon vanilla extract
For the crumble:
1 and ¾ cups all-purpose flour
1 and ½ cups rolled oats
1 cup packed brown sugar
½ teaspoon cinnamon
1 cup unsalted butter, cold & cut into ½-inch pieces
Make the filling:
In a small heavy saucepan, combine rhubarb, sugar and water. Bring to a boil over medium heat. Reduce heat to low and simmer until rhubarb is tender and beginning to break down, about 8-10 minutes. Stir in strawberries and vanilla and cook for 5 more minutes. Set aside to cool slightly.
Make the squares:
Preheat oven to 325 degrees F. Butter or spray a 9×9-inch baking pan.
In a large bowl, combine flour, oats, sugar, cinnamon and salt. Add the cold butter, and use your fingers to rub the butter into the dry ingredients until the mixture looks like coarse crumbs about the size of small peas. Reserve 2 cups of the oat mixture and set aside.Press remaining oat mixture into prepared pan.
Spoon 1 and ½ cups of the strawberry-rhubarb mixture onto the bottom crust and spread evenly overtop (Note: you may have extra filling; reserve for another use). Sprinkle the reserved oat mixture over the filling.
Bake about 50-55 minutes, until the top is lightly browned and the filling is bubbling. Let cool on a rack for 2 hours before cutting into squares and serving. The finished squares will keep covered at room temperature for 2-3 days.