I would guess mushrooms are a bit tricky because they have so much water in them.
If you like Portabellas though, you HAVE to try this:
Mix together some good olive oil, balsamic vinegar, and roughly chop a few cloves of garlic. Throw that in a large ziploc w/ 3-4 portabellas, stem removed, and a couple of grinds of fresh pepper.
3 hours is good, but overnight is really how you want to do this. Take them out and put a LIGHT sprinkle of salt on each one and give them a good 5 minutes, gill side down, on a fairly hot grill. Flip, and another 3-5 mins cap side down. you're looking for a bit of char on them.
I slice them on a cutting board and get about four, 1-inch pieces per 'shroom. They are so juicy and good, they never even make it to a serving platter, no joke. These things are incredible!
I've experimented w/ fresh herbs and such too, but I find simplest is best with these. Enjoy. [

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