A grilled cheese should be perfectly crispy on the outside and soft and gooey on the inside. A grown-up version of my favorite childhood sandwich, this one is inspired by both an Italian antipasti plate and an English cheese toastie with its sweet and sour Branston pickle. First, I prepare a relish by adding dry-seared mushrooms (they retain a little more freshness than cooked in butter or oil), chopped pepperoncini, Castelvetrano olives, and a dash of citrus. Then I sandwich thin slivers of extra-hot cheddar between thick slices of sourdough miché. The oiliness of the cheese and the earthy note of the mushroom pair perfectly with the tangy peppers and salty olives. The relish is a star in its own right – try it as a dip stirred into labneh or as a spoon with grilled fish or on top of roasted vegetables.
Prepare the Relish: Heat a cast-iron skillet over medium-low heat. Add the chopped mushrooms and dry sauté for about 4 minutes. Stir constantly to prevent sticking (they give off moisture as they cook).
In a bowl, mix together the mushrooms, pepperoncini, olives, lemon zest, and lemon juice.
Make the sandwiches: Cut eight 1 cm thick slices of bread. Using a mandolin or a very sharp knife, slice the cheese very thinly. Place the bread on a cutting board or sheet and brush one side of each slice with 1/2 tablespoon of butter. Flip 4 slices, buttered side down, and top each with 4 tablespoons of relish, followed by evenly spaced cheese slices. Place the remaining 4 slices of bread on top, buttered side up.
Heat a cast-iron skillet over medium-high heat. Bake the sandwiches until golden brown, about 4 minutes per side. Occasionally, use a spatula to press down firmly on each side and turn at least twice. Sprinkle both sides of each sandwich with salt. Serve hot.