Here are the approximate directions for making the sandwich (Makes 2):
Thinly slice one shallot and 6-8 oz white or cremini mushrooms. Sauté the shallot and mushrooms together in a mixture of butter and oil until they are golden and very soft. Add 3 Tbsp of Balsamic Vinegar and 1 tsp of Red Wine Vinegar to the mushroom mixture and reduce until the liquid has almost evaporated and everything is nicely glazed. Salt and pepper to taste.
Grate about 2 cups of Fontina or other preferred cheese.
Butter four slices of your favorite bread. With the heat off, arrange two slices–butter side down– in a non-stick frying pan. Arrange the mushroom mixture on top of the bread, then add the cheese, then the other pieces of bread–butter side up. Turn on the heat and heat through (3-5 minutes per side). The sandwich is messy and delicious, so cutting in half is recommended.