from smitten kitchen: http://smittenkitchen.com/2007/05/crumbling-crisp-convictions/
Yields 6 to 8 servings.
For the topping:
1 1/3 cup flour
1 teaspoon baking powder
3 tablespoons sugar
3 tablespoons Demerara sugar (or turbinado sugar aka Sugar in the Raw)
Zest of one lemon
1/4 pound (1 stick or 4 ounces) unsalted butter, melted
For the filling:
1 1/2 cups rhubarb, chopped into 1-inch pieces
1 quart strawberries plus a few extras, hulled, quartered
Juice of one lemon
1/2 cup sugar
3 to 4 tablespoons cornstarch (some commenters found the flour option a little too, well, floury so this has been updated)
Pinch of salt
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.