The problem you are having is the moisture from the fruit is preventing the topping from cooking. I would cook the peaches seperately in a pot on the stove top and bake the topping in the oven on its own. This will eliminate your problem completly and you will have much better control over how the topping comes out. You can assemble the dish afterwards depending on which kind of presentation you prefer.
8/16/10 at 3:48am