In a large heavy pan over medium heat, add 2 Tbsp butter and 2 Tbsp oil. Once oil is hot, add mushrooms in a single layer, cut-side-down. Sprinkle with garlic salt and pepper. Sautee until bottoms are browned (5 min), then flip over, season again with garlic salt and pepper and sautee until browned on the second side (5 min).