Whisk the sugar and the flour together in a large mixing bowl. Add butter, cutting it in with a pastry blender or a fork until the mixture is crumbly. Add oats, cinnamon, and pecans (if using); blend until clumps form.
FOR THE FILLING
Approximately one pound of fresh fruit. But, if you have more or less, that’ll work too. If you have a lot less you can cut the topping in half.
If your fruit is particularly tart, you may want to add ¼ to ½ cup granulated sugar, but I rarely, if ever, do this step. If your fruit is particularly liquidly, like strawberries can be, and you want to thicken it, stir in 1/8 cup of quick-cooking tapioca. Some recipes will tell you to add the sugar and tapioca to the fruit and wait ½ hour for the juices to start; I don’t usually do this step. I like the way the fruit comes out when I just place it straight into the oven.
Preheat oven to 350°.
Place the fruit in a 9x9 or 13x9-inch dish (depending on how much fruit you have), and top with the crumble. Bake in a 350° oven until filling is bubbling thickly and topping is brown and crisp, about 30-40 minutes, depending on how much fruit you have. Let cool at least 15 minutes before digging in (otherwise you’ll burn your tongue; trust me on this one!).