This is a sandwich I frequently have a craving for. My husband, who will eat just about anything vegetarian and in huge quantities, does not really like portobellos unless they are disguised in something. So when I make this it is just for myself. I like the mushrooms just topped with mustard or mustard and cheese, but you can add tomatoes or onions or avocados. Really, anything you like on sandwiches.
I clean a portobello cap and lightly brush it with olive oil. I season it with salt and pepper and grill it on an indoor grill pan or on the outdoor grill. Of course, you can also pan fry it. I cook the portobello until it is tender and a bit mushy. I slice the mushroom and put it on a french roll with some spicy brown mustard and some thinly sliced Gouda. I heat the sandwich in the oven for a couple minutes until the cheese melts. Really yummy!