[Aliantha]

just sprinkle more flour into the apples when you lay them out. Also, remember that sugar turns into a liquid when it's heated. You could use less sugar if you've got nice tart apples. Instead of white sugar, try using brown sugar. It goes more syrupy when it melts, thus not causing so much excess moisture.

Another way to cook an apple pie is to stew the apples first and thicken with regular flour or cornflour. The only problem with cornflour as a thickener in this process is that once it's re-cooked, it loses some of it's thickening power, so if possible, either reduce the liquid content in the apples, then the thickening is only required for the smallest amount of moisture, or get prebaked pie crust so that you can simply add the apple mixture, then put a crust on top and then grill it. That way you don't reheat the whole inside of the pie. Just the top.

All in all though, I think you'll find the brown sugar the way to go. Because it's less refined, it's sweeter and you need less anyway, thus less moisture.