Put the marshmallows in a 10-inch cast-iron skillet or other ovenproof saucepan or skillet; they can be slightly layered on top of each other. Place a sheet of aluminum foil on top to cover the skillet. Bake on the middle shelf of the oven for about 15 minutes.
Remove the skillet from the oven, remove the foil and raise the oven temperature to low broil. Put the skillet back in the oven and broil until the marshmallows are toasted and browned, 1 to 2 minutes. (Do not walk away from the oven. The marshmallows can burn fast.)
Wrap the handle of the skillet with an oven mitt or thick kitchen towel (the handle is extremely hot), then serve the marshmallows alongside the bar condiments, letting your guests build their own creations with the cookies, graham crackers, nuts, sauces and fruit.