Fruit Crumble

Heat oven to 375°F with rack in center position. Coat a 9-by-9-inch or 11-by-7-inch baking dish with butter; set aside.

Prepare the fruit filling. If necessary, dice the fruit into bite-sized pieces, removing any stems, seeds, or inedible parts. Toss the fruit with sugar, lemon juice, cornstarch, and any spices in a large bowl. Use more sugar and less lemon juice when cooking with tart fruits, like rhubarb and blackberries, and less sugar but more lemon juice for sweet fruits, like peaches and plums. (Just taste the fruit you’re using and adjust to taste.) Use more cornstarch with very juicy fruits like plums and less with firm fruits like apples. Pour the fruit filling into the baking dish. Transfer the fruit filling into the baking dish.

Prepare the crumble topping. Whisk the flour, sugar, baking powder, cinnamon, and salt together in a large bowl. Cut the butter into a few large pieces, and toss these in the dry ingredients. Using your fingers, a fork, or a pastry cutter, work the butter into the dry ingredients until large, heavy crumbs are formed. Scatter the crumble topping evenly over the fruit.

Bake until the fruit juices are bubbling around the edges of the pan and the topping is firm to the touch, 30 to 35 minutes. Let the crumble cool for at least 15 minutes before serving. If transporting to a picnic or party, let the crumble cool completely to give the fruit filling time to set.
To make a crisp: Add ½ cup of old-fashioned rolled oats or ½ cup chopped nuts (or both) to the topping.