Preheat oven to 375F.
Streusel Topping:
Combine flour, baking powder, sugar, lemon zest, salt and butter. Combine with hands until mixture is mixed and the size of peas. Our whirl in a food processor. Set aside in refrigerate.
Filling:
Combine rhubarb, strawberries, lemon juice, sugar, cornstarch and salt in a shallow gratin dish or 9-inch pie plate. Top with streusel topping. Place dish on a cookie sheet to catch drippings. Bake 30-45 minutes or until bubbly.