Use a large, non-stick saute pan, and heat the olive oil.

Lay the mushrooms in a single layer in the pan, and saute until the first side is well browned. Turn over and brown the other side as well.

Drizzle the balsamic vinegar over the slices and shake the pan to get each slice covered. Cook until the pan is dry again, but be careful not to let the mushrooms burn. Turn off the heat, remove the mushrooms from the pan and give them a little sprinkle of sea salt. Wipe the pan out with a paper towel.

To assemble the sandwich, very lightly butter one side of the bread slices.

Turn the slices over and spread the mustard over the other side.

Place one slice of bread for each sandwich butter side down in the saute pan.

Layer the cheeses and mushrooms on the bread, with the mushrooms in the middle. Sprinkle on the parsley. Top with the second bread slice, mustard side facing in.

Turn the heat to medium and cover the pan loosely to keep the heat inside - this will help help the cheese melt more evenly.

Cook for 5 minutes and check to see if the bottom is toasted to your liking - if not, leave another minute or two.

Turn sandwich over carefully and cook another 5 minutes or so, until the second side is toasted as well.

Serve immediately.