Heat a large skillet over medium heat and add the olive oil. Once heated, add the sliced mushrooms and sauté for about 3 minutes, stirring occasionally.

Sprinkle a pinch of kosher salt over the mushrooms. Sauté for an additional 3 minutes, until golden brown, then add black pepper and remove from the pan to a small bowl.

In a separate large skillet or griddle, heat the truffle oil over medium-low heat. Spread the butter evenly on one side of each slice of sourdough bread.

Place half of the slices of bread, butter-side down, on the skillet. Distribute the Fontina cheese evenly over these slices, then add the sautéed mushrooms and arugula on top of the cheese. Cover with the remaining slices of bread, butter side up.

Cook for about 4 minutes until the underside is golden brown, then carefully flip the sandwiches using a spatula and press down gently. Cook until the second side is golden brown and the cheese is melted.

Cut each sandwich in half and serve immediately, preferably with a side of tomato soup.