Whisk together oil, butter, vinegar, soy, rosemary, and garlic. Toss mushrooms in the
liquid and marinate for two hours, up to overnight.
Thirty minutes prior to cooking, ignite the charcoal in a chimney starter. When coals are
glowing and just ashed over, dump into a pile onto one side of the grill, then rake a few
coals from your original pile to the opposite side of the grill floor, creating one high-stacked
hot zone for searing (or sautéing) and one cooler zone for roasting. For every hour of
cooking, add a half-chimney of coals.
Meanwhile, add mushrooms and marinade to a well-seasoned and oiled cast iron pan. Place pan directly into the smoldering charcoal embers, stirring intermittently until liquid
reduced by ½. Move pan to indirect heat by repositioning the pan away from the coals or
above the coals on an elevated grill grate. Remove from heat when the mushrooms are
tender and all liquid absorbed.
Rest for 5 minutes and season additionally to taste. Plate atop a bed of quinoa and fresh