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