Discard dark or soft spots from mushrooms. Quickly swish mushrooms in cool water to rinse, then drain. Cut into 1/2- to 3/4-inch-thick slices. Melt butter in a 10- by 15-inch rimmed pan. Turn mushrooms over in butter; arrange side by side. Bake in a 400° oven about 10 minutes. Turn slices over; bake until browned, 10 to 15 minutes more. If making ahead, let cool in pan, then reheat in oven 4 to 5 minutes. Season with salt and pepper.