Make the topping. In a mixing bowl combine flour, baking powder, sugars, lemon zest, and the melted butter. Mix until clumps form; set aside.
Make the filling. In a deep 9-inch pie-plate combine rhubarb, strawberries, lemon juice, sugar, cornstarch and a pinch of salt. Mix until incorporated.
Assemble. Add the previously prepared topping over the fruit mixture.
Bake. Place the pie plate on a foil-lined baking sheet and bake for 45 minutes, or until the topping is golden brown and the fruit is bubbly.
Rest and serve. Remove from oven and let stand 8 to 10 minutes. Serve.
Notes
Thanks to the wonderful blog, Smitten Kitchen, this really is the best version of strawberry-rhubarb crumble pie that I have tried. I have been making it for the past three years, and as soon as rhubarb appears, this is the first recipe that I make with it.