Mix berries, sugar, lemon juice and zest, and flour in a large bowl. Preheat oven to 400 degrees F and grease a 9x13-inch glass or ceramic baking dish.

To make topping, combine flour, brown sugar, oats, cinnamon, and salt in the bowl of a food processor. Add butter, then pulse the machine repeatedly, in 2- to 3-second bursts, until the mixture is clumpy, like damp crumbs. Transfer the berries to the baking dish, then spread the crumbs evenly over the fruit. Bake for 30 minutes, until bubbly hot. Serve at any temperature, although it is best to let it cool at least 10 minutes.