There are two secrets to making a crisp. The first is to work with cold butter for the topping, and if you're using frozen berries to keep them that way until the dish goes into the oven.
Pre-heat the oven to 350°F and have a 7x11-inch baking dish or equivalent-sized casserole on standby.
Mix the oatmeal, sugars and cinnamon in a small bowl. Chop or grate the frozen or very cold butter into small pieces. Use a small amount (just enough!) of the butter to grease the baking dish. Stir the rest of the butter into the oatmeal mixture and place the bowl in the refrigerator.
Tumble the frozen fruits into a large mixing bowl. Sprinkle the sugar and cornstarch over top. Using a spatula, stir until the berries are evenly coated by the dry ingredients.
Place the prepared berries into the baking dish. Top evenly with the crumble (there's a lot, but don't worry, it all cooks down).
Place the dish onto a baking tray in case any of the fruit juices run over. Put into the oven and bake for 40 to 45 minutes, or until the topping has browned and the juices are bubbling around the sides of the dish.
Cool on a baking rack. Serve warm or at room temperature. If you'd like to fancy things up, serve with a scoop of vanilla ice cream. Or, go all out and layer with yogurt (plain Green yogurt is my choice) into a simple glass tumbler or a pretty parfait glass.
Berry Cherry Crisp https://urbancottagelife.com/berry-cherry-crisp-%e2%9d%a7-easy-any-time-of-year/