Prepare fruit and place in a large bowl. Whisk sugar and cornstarch until well combined, then add to fruit. Toss until fruit is coated and dry ingredients are no longer visible. Spoon into a 13 x 9 pan and set aside.
Mix together all of the topping ingredients until well combined and crumbly. Sprinkle evenly on top of fruit.
Bake at 350 F/177 C for 45 minutes, until crisp is bubbly around the edges. Let cool. Serve warm (with whipped cream or ice cream, if you like) or completely cooled. Stores well covered at room temperature or in the refrigerator.
Notes
*If using only rhubarb, simply leave out the extra fruit and follow the rest of the recipe as written.Adapted from Taste of Home's Rhubarb Crisp.