1. Heat oven to 375°F. Prepare topping: In a mixing bowl, combine flour, baking powder, sugars and lemon zest and add the melted butter. Mix until small and large clumps form. Refrigerate until needed.
2. Prepare filling: Toss rhubarb, strawberries, lemon juice, sugar, cornstarch and a pinch of salt in a 9-inch deep-dish pie plate. (I used an oval dish this time, because they fit better in the bottom of a shopping bag.)
3. Remove topping from refrigerator and cover fruit thickly and evenly with topping. Place pie plate on a (foil-lined, if you really want to think ahead) baking sheet, and bake until crumble topping is golden brown in places and fruit is bubbling beneath, about 40 to 50 minutes.