Place the rhubarb, strawberries, sugar, lemon zest, balsamic vinegar and black pepper in a sauté pan. Cook over medium heat until mixture comes to a simmer and begins to breakdown and thicken, 5-6 minutes. The rhubarb will start to break down, try to keep it a little chunky for a more rustic filling. Set aside and allow to cool to room temperature.
Preheat oven to 400˚F. Lightly coat a baking stone or 9 inch deep baking pan with cooking spray.
In a mixing bowl beat the flour, graham cracker crumbs, sugar, egg and baking powder until it becomes a crumbled mixture.
Sprinkle three quarters of this mixture on bottom of the pan. Spoon the strawberry rhubarb mixture on top and sprinkle remaining crumb mixture on top. Melt the butter, drizzle over the top and sprinkle with finishing sugar. Bake for 30-35 minutes until golden brown.