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.

Ingredients

• 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 ½ pound
• 1 tablespoon olive oil
• 2 tablespoons soy sauce
• 2 tablespoons balsamic vinegar
• 1 minced garlic clove
• 8 clean portobello caps

Instructions

1. 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.
2. 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.)
3. In a foil pan pour in the olive oil, soy sauce, balsamic vinegar and minced clove of garlic.
4. Add the portobello caps upside down and grill on the BBQ for 15 minutes uncovered.
5. Add some of the cheese on the portobellos and continue to grill for 3 minutes.
6. Spray the vegetable pan with olive oil spray. On the vegetable grill pan place the four slices of of bread that have been buttered on one side, butter side down. Add to the bread the caramelized onions and the rest of the grated cheese and cook for 10 more minutes.