Step 1 Place a large cast-iron skillet in the middle of the oven and heat it to 500 degrees. This will take about 30 minutes.
Step 2 In the meantime, prepare the mushrooms. With a damp towel wipe off any dirt. If you must rinse them, do so gently and pat dry.
Step 3Create various shapes: Slice crimini and button mushrooms in half. Remove the bottom tuft of the king oyster mushrooms and discard, then slice lengthwise into four or five long pieces before cutting down the middle so you have short rectangles. Rip the oyster mushrooms into halves by hand. Slice the portobello into quarters or eighths as desired.
Step 4Remove the pan and add all the mushrooms. Return it to the oven for five minutes. Then remove again, and stir the mushrooms so that any sides unexposed to the hot iron get a good sear. Cook for an additional five minutes.
Step 5Remove the pan and add the shallots and butter, and toss. Return to the oven for another 5 minutes.
Step 6Final removal: Add the garlic and parsley, and set aside for 3 to 5 minutes to give the flavors time to marry. Salt generously and pepper profusely. This makes about 4 cups of mushrooms, more than you'll need for the remainder of the recipe.