This is a traditional patty melt in all ways save the fact that the beef has been swapped out for roasted portobello-mushroom caps. It otherwise hews closely to the recipe served at Tiny Naylor’s drive-in restaurant in Los Angeles in the 1950s, and to the ones used in coffee shops and diners across the country. But those mushrooms! Roasted in the oven in a marinade of oil, balsamic vinegar, soy and garlic, they take on immense flavor and density, and provide a terrific foil to the caramelized onions, Swiss cheese and butter-griddled rye bread. – Sam Sifton of NY Times Cooks
- 2 to 4 tablespoons unsalted butter
- 2 large onions, peeled and thinly sliced
- Kosher salt and freshly ground black pepper to taste
- 8 slices seeded rye bread
- 8 slices Swiss cheese, approximately 1/2 pound
- 1 tablespoon olive oil
- 2 tablespoons soy sauce
- 2 tablespoons balsamic vinegar
- 1 minced garlic clove
- 8 clean portobello caps
- Caramelize onions. Melt 2 tablespoons of the butter in a large skillet set over high heat. When it foams, add the onions and sprinkle with salt. Do not stir immediately. Wait 1 minute, then begin to stir frequently over high heat for 5 minutes or so, or until the onions have released some liquid and started to become translucent.
- Reduce the heat to medium, and cook, stirring often, for what will seem like a very long time, until the onions are fully melted and dark golden brown, approximately 30 to 40 minutes. Remove onions, and set them and the skillet aside. (You can caramelize the onions a day ahead of time and reheat slowly when you’re ready to cook.)
- Take a small baking dish, and put into it olive oil, soy sauce, balsamic vinegar and minced clove of garlic. Add portobello caps, then cover with foil, and roast in a 400-degree oven for 30 to 45 minutes, turning once.
- Set the skillet to medium heat, and put four pieces of rye bread into the butter that remains from cooking the onions. Top each piece with a slice of cheese, then two portobello caps, then some of the caramelized onions, and finally another slice of cheese and another slice of rye. Use the spatula to press down on the packages, and after about a minute or so, carefully turn over each patty to begin to brown the other side. (You may need to add the additional butter.) Cook until the cheese is fully melted and the bread is golden brown and crisp on both sides. Slice in half before serving.