Heat the oven to 375 degrees F. Grease the bottom and sides of an 8 inch square baking pan.
Combine the rhubarb and sour cherries in a bowl. Sprinkle with white sugar and 1 tablespoon flour. Stir to coat the fruit. Set aside.
Meanwhile, combine the remaining flour, brown sugar, oats, salt, and nutmeg in another bowl. Stir well. Drizzle in the melted butter and stir to combine.
Once the sugar has pulled some juice from the rhubarb (about 5-10 minutes), spread the fruit in the prepared pan. Sprinkle the flour and oat mixture over the fruit.
Bake for about 35-40 minutes, until the topping is golden brown. Serve on its own or with cream, vanilla Greek yogurt, and ice cream.
Notes
Really, any combination of 5 cups of rhubarb and cherries will work. Use what you have!