Heat oven to 375°F.
Prepare topping: In a mixing bowl, combine flour, baking powder, sugars, lemon zest, a pinch of salt, and add the melted butter. Mix until small and large clumps form. Refrigerate until needed.
Prepare filling: Toss rhubarb, strawberries, lemon juice, sugar, cornstarch and a pinch of salt in a 9-inch deep-dish pie plate.
Assemble and bake: Remove topping from the refrigerator and cover fruit thickly and evenly with it. Place pie plate on a baking sheet, and bake until crumble topping is golden brown in places and fruit is bubbling beneath, about 40 to 50 minutes.
