Preheat the oven to 350 degrees F. Butter or spray a 13x9" baking pan then line with a sling of parchment paper covering the bottom and two sides and extending upward from the pan (to give you handles.)

In a large bowl, combine the cherries, rhubarb, lemon juice and sugar. Set aside.

In the bowl of your mixer, beat together softened butter and sugar until fluffy. Beat in grated lemon zest. Add eggs one at a time until well incorporated. In a separate bowl, whisk together flour, baking powder, salt and ground ginger. Add flour mixture alternatingly with sour cream. Turn out onto parchment-lined baking pan and spread evenly to the corners with an offset or silicone spatula. Cover evenly with the fruit mixture.

In a small bowl, whisk together flour, brown sugar, cinnamon and salt then whisk in butter until a loose crumb forms. Sprinkle evenly over fruit and cake. Bake for 50 to 60 minutes until a toothpick inserted in the center comes out dry. Cool for 20 minutes before lifting the cake out by the sling and cooling completely on a wire rack.

Cut into squares and place each on a square of parchment to store or serve. It keeps, tightly covered, at room temperature for a few days or stored in the refrigerator.