Preheat the oven to 400 degrees F.
For the crisp: Mix together the flour, brown sugar, cinnamon and salt in a bowl. Using a pastry blender, 2 knives or a food processor, cut in the butter. Stir in the oats.
For the fruit: Toss the fruit with the cornstarch, granulated sugar as needed and the lemon zest if using.
Transfer the fruit mixture to a baking dish and top with the crisp mixture. Bake until the middle is warmed through and the top is crispy and golden brown, 50 to 55 minutes.
The ratio of crisp to fruit is fairly flexible--use more or less based on your preference. Larger batches of the crisp may take more time to bake, while smaller batches may take less time. Watch closely and adjust the baking time accordingly.
This recipe has been updated and may differ from what was originally published or broadcast.