To make the topping
In a bowl, mix together the flour, brown sugar, granulated sugar, nuts, and cinnamon. Work in the butter until the mixture resembles dry oats. The mixture should just hold together and look crumbly.
To make the filling
Heat the oven to 350° F. Peel, core, and quarter the apples. Cut each quarter into four chunks; you should have about 5 cups. Trim the rhubarb and cut it into 1-inch-long pieces 1/2 inch wide; you should have about 5 cups of rhubarb. In a large bowl, toss the apples and rhubarb with the sugar, flour, and cinnamon until well coated. Transfer the apple and rhubarb mixture to a 2-qt. baking dish and sprinkle the topping over the fruit. Bake until the topping is golden brown and the fruit is bubbling, 1 hour to 1 hour, 15 minutes. Cool slightly before serving.
nutrition information (per serving):