Prepare the grill and preheat it for medium-high temperature. Let the surface preheat for about 5 minutes and place the cast iron skillet onto the grill. (If you don’t have a cast iron skillet, use another grill-safe pan.) Let the skillet preheat for about 5 minutes over direct heat.
Rinse whole mushrooms under cold water and pat them dry with a paper towel.
Cut large ones in fours or halves but small ones can be left uncut.
Melt butter in the skillet and add mushrooms.
Sprinkle salt, pepper, and thyme, over the top and and mix everything well.
Let mushrooms cook, stirring them from time to time, until done.