Mushroom, shallot, and melted cheese is a match made in sandwich heaven. Shallots and balsamic vinegar add a bit of sharp bite, and fontina has natural notes of woodsy earthiness, making it the perfect thing to melt on top.
Heat a saute pan until hot. Add the mushrooms, lower the heat to medium, and sprinkle with salt and black pepper. Cook until mushrooms release liquid and start to brown and wilt, 3 minutes. Add 1 teaspoon oil and continue cooking until mushrooms are softened. Add the shallots and saute for 3 minutes longer, adding more oil if needed. Pour in the vinegar and cook until absorbed, about 2 minutes. Stir in the herbs.
Spread some of the oil onto 1 side of the bread slices, and sprinkle lightly with salt. Heat the remaining oil in a large saute pan. Lay the cheese over 2 of the bread slices, and top evenly with the mushrooms. Add all 4 bread slices to the pan. Cook over medium heat for 2 minutes, and then cover and continue cooking until cheese is melted, about 2 minutes longer. Assemble and serve.