Preheat your oven to 350°F (175°C). Grease a 9-inch baking dish or pie plate.
Prepare the Fruit Filling: In a large bowl, combine the mixed fruit with white granulated sugar, 2 tablespoons of flour, and cinnamon (if using). Toss until the fruit is evenly coated. Transfer the fruit mixture to the prepared baking dish, spreading it out evenly.
Make the Crumble Topping: In a separate bowl, mix the rolled oats, ½ cup of flour, brown sugar, and a pinch of salt. Add the cold butter pieces, using your fingers or a pastry cutter to mix until the mixture is crumbly and resembles coarse crumbs.
Assemble: Sprinkle the crumble topping evenly over the fruit filling in the dish.
Bake: Place the dish in the preheated oven and bake for 30-45 minutes, or until the topping is golden brown and the fruit filling is bubbly. Check around 30 minutes and cover with foil if the topping is browning too quickly.
Serve: Allow the fruit crisp to cool for at least 10 minutes before serving. Serve warm, ideally with a scoop of vanilla ice cream.