Combine 5 cups rhubarb with 1 –1 1/2 cups sugar, (depending on sweetness desired) and 5 Tablespoons cornstarch and cook over medium heat until thickened. Remove from heat and add 2-3 cups sliced strawberries.
Cut 1 cup cold butter into 1 1/2 cups brown sugar, 2 cups oats, 1 1/2 cups flour, 1/2 teaspoon salt, 1 teaspoon baking powder. Spread half of crumb mixture into a greased 9x13 baking dish.
Spread rhubarb-strawberry mixture over bottom layer of crumb mixture, then top with remaining crumb mixture and gently pat it down.
Bake in preheated 375 degree oven for 40 minutes until top is golden brown.
Serve warm with your favorite vanilla ice cream.
A perfect summer dessert!