Place the pineapple in medium pot; pour in 1-1/2 cups of the pineapple juice. Bring to a simmer. Place the remaining pineapple juice and the cornstarch in a small bowl and whisk to combine. Add to the pot and return to a simmer. Cook until a clear sauce forms around the pineapple, about 2 minutes. Spooning the mixture into an 8-inch x 8-inch baking dish. Preheat the oven to 350 degrees F. Combine the ingredients for the topping in a medium bowl; spread over the pineapple. Bake for 30 minutes, or until the top is golden and filling is bubbling.
Recipe Options: Make the crisp in advance and bake later. To do so, make the filling and allow it cool to room temperature in the pan; cover and store in the fridge. Combine the topping in a bowl and cover and store at room temperature until needed. When ready to bake, spread the topping on the filling and bake as directed, adding 5 to 10 minutes more baking time, as you will be starting from cold. Make pineapple-mango crisp by replacing half the pineapple with peeled cubes of ripe mango.