"Mom gave me the original recipe which is kind of a cross between a coffee cake and dessert bar, using a strawberry-rhubarb combination, but I changed it to one with cherry, which I think gives it a richer flavor."
Combine rhubarb and lemon juice in a large saucepan over medium-low heat; cook, stirring often, for 5 minutes.
Mix 1 cup sugar and cornstarch together in a bowl; stir into rhubarb mixture. Cook and stir until rhubarb mixture is thickened and bubbling, about 5 minutes. Stir in pie filling; remove from heat and cool filling, about 10 minutes.
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Combine 3 cups flour, 1 cup sugar, baking powder, baking soda, and salt in a large bowl; cut in 1 cup cubed butter until mixture resembles fine crumbs.
Beat buttermilk, eggs, and vanilla extract together in a separate bowl; add to flour mixture and stir just until batter is moistened.
Spread a little more than half of the batter into the prepared baking dish; top with cherry-rhubarb filling. Drop teaspoonfuls of remaining batter over filling.
Combine 1 1/2 cup sugar and 1 1/2 cups flour in a bowl; cut in 1/2 cup cubed butter until mixture resembles coarse crumbs. Sprinkle mixture over batter.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, 40 to 45 minutes.