If you want a lovely brown crust on your mushrooms, use high heat (preheated pan of course) and a reasonable amount of oil (or a combination of oil and butter), place your mushrooms in a thin, even layer over the entire cooking surface (it's ok if you have a tiny little bit of stacking - they don't have to be perfectly arranged), and let them sit long enough on one side to form that crust before you redistribute them, then repeat.