What is the best way to clean/prep a button mushroom?
Well, first I would say don't wash with water. Mushrooms are very porous and will absorb the water like a sponge. Use a mushroom or fine bristle brush and delicately brush the dirt off. Slice the end of stem off just a little bit to get the hard or woody end off. From here you can use it whole, quarter it for larger chunks or slice it by turning over onto it's top and slicing through the stem. Or you can remove the stem and and slice or leave whole to use whole and stemless. or remove the stem so that you have a cavity to fill and chop the stem up and add to whatever you would like to fill the cavity with for stuffed mushrooms.
I just wipe them off with a paper towel, and pull out the stems, then do whatever I'm gong to do with them.
I peel them, English style. You gently pull the lose membrane from under the cap and peel up. It's a bit putzy, but looks great!
I rinse them under a trickle of warm running tap water, rubbing them gently with my fingers until I'm sure every last bit of growing medium has been removed. Eating mushroom dirt ranks right up there with chomping on eggshells. After rinsing, I pat them dry.
For mushrooms used in cooking, I use a clean, dry, very soft dishcloth, which I then moisten slightly with a tablespoon or two of water; then I use the damp place to wipe off any dirt. The texture of the cloth works a bit better than paper towels (which I used for years for this purpose). Then I cut off about 1/4 inch of the stem end. ;o)