1. Wash and clean the mushrooms.
2. Place the mushrooms (whole or cut into slices) in a baking dish. Add olive oil and sprinkle with a little bit of salt.
3. Bake at 300 F for about 5-10 minutes. In the meantime, cut one whole onion into small slices. Add to the mushrooms after 10 minutes (the mushrooms should have shrunken but 30-40% by then).
4. Add the sliced onion to the mushrooms and bake for 10 more minutes.
How to Make Baked Mushrooms. eHow.