Step 1

In a large (ideally 12-inch wide cast iron) skillet over high heat, melt 2 tablespoons of the butter. Add the olive oil, mushrooms, onion and salt. Fry, stirring frequently, until the onions and mushrooms caramelize and crisp, 10 to 15 minutes, adjusting the heat as necessary if they start to burn. Remove from the heat and stir in the Worcestershire.

Step 2

Assemble the sandwiches: Lay 4 slices of bread onto a flat surface. Place at least 1 slice of cheese on each slice of bread. Top with a quarter of the onion-and-mushroom mixture, followed by at least 1 slice of cheese and the remaining slice of bread.

Step 3

Wipe the skillet out and place over medium heat. Melt the remaining 4 tablespoons of butter, and fry the sandwiches, in batches if necessary, until the cheese has melted evenly and both sides are crisp, 2 to 3 minutes per side. Slice in half and serve hot, with pickles on the side, if desired.