By popular (well, someone asked) demand:
1/2 peck apples, see below
1 stick butter
1/2 cup flour
1/2 cup sugar
1/2 cup rolled oats
Use the butter paper to butter a three quart rectangular baking dish. Preheat the oven to 350.
Peel, core, and slice the apples and put them in the baking dish. Shake in some cinnamon if you want. If you do this more than once, get an apple peeler/corer/slicer doozit like the one linked below, which will let you do all the apples in a jiffy. They should heap up somewhat above the side of the baking dish. Put them in the oven to bake for a while.
Combine the butter and sugar with a pastry blender or a fork, then mix in the flour and the oatmeal and use your fingers to mix it into a big lump about the size of a softball.
After the apples cook for a while and soften, take the dish out of the oven and use a fork to push them down and even them out level with the edge of the dish. This will also dry them out somewhat.
Take bits of the dough about the size of a pingpong ball, flatten it between your palms as flat as you can, and lay the flattened dough on the apples. Repeat until you've used it all, covering all the apples as much as you can. The dough will expand a little as it cooks, so small gaps don't matter.
Put it back in the oven and cook for half an hour, or until the dough puffs and browns, and the apples are bubbling. The topping seals the apples, so they won't dry out at this stage.
Serve warm or cool, with or without ice cream.
IMPORTANT: The success of this dish depends entirely on the quality of the apples. If you use grocery store delicous or golden delicious or mcintosh apples, it'll be watery, mushy, and tasteless. Granny smith are sour and boring. I've been using mutsu (aka crispin) apples mixed with other kinds. If you can find Bramleys, they're great.
Example of an apple peeler doozit