Total time: About 25 minutes, plus marinating time for the onions
Servings: Makes 2 sandwiches
This recipe is adapted from chef-owner Mark Peel of Campanile restaurant in Los Angeles.
½ cup Spanish or brown onion, thinly sliced
¼ teaspoon kosher salt
¼ teaspoon freshly ground black pepper
1 tablespoon best-quality olive oil
1 tablespoon white wine vinegar
4 slices country white bread
4 tablespoons salted butter, melted
5 ounces Gruyere cheese, coarsely grated
4 teaspoons stone-ground mustard
In a small bowl, mix together the onion, salt, pepper, olive oil and vinegar. Cover and let sit for at least one hour.
Heat a cast-iron pan or griddle over low heat. Brush one side of each slice of bread with the melted butter. Place two slices of bread, butter-side down, on a cutting board. Spread half of the grated cheese evenly over the bread. Drain the excess liquid from the marinated onions and scatter them evenly over the cheese-topped slices.
Distribute the remaining cheese evenly over the onions. Spread the mustard over the unbuttered sides of the remaining 2 slices of buttered bread. Place these, mustard-side down, on top of the slices spread with cheese.
Using a spatula, place one of the sandwiches into the hot pan and weight with another heavy skillet or a steak weight. Cook until golden brown, about three to four minutes. Flip the sandwich and weight it with the pan or steak weight, and cook until golden brown, another three to four minutes.
Each sandwich: 745 calories; 29 grams protein; 37 grams carbohydrates; 55 grams fat (29 saturated); 138 milligrams cholesterol; 1,131 milligrams sodium.