- Melt 2 tablespoons butter in a stainless steel pan or cast-iron skillet over medium heat.
- Add onions, stirring gently to coat with butter.
- Caramelize the onions by turning heat down to low and stirring every 10 minutes or so. Be careful not to let them burn! When stirring, scrape up any browning that forms on the bottom of the pan. Exact cooking time will vary based on the amount of onions, their liquid and sugar content, and their age, but will be about 30−45 minutes. They are done when they are a rich brown color. If there is a lot of liquid, drain it off to allow for better browning. When finished caramelizing, let onions cool to room temperature before using in the sandwich to prevent them from being too wet. Note: If you’re not using the onions immediately, let them cool in the pan, then transfer to a storage container. Caramelized onions can be kept refrigerated for about 1 week or frozen for up to 3 months.
- To assemble the sandwich, spread butter on the outside of two pieces of multi-grain bread. Place a layer of Gruyère cheese on one slice of bread. Spread a generous spoonful of caramelized onions on top of cheese and place second piece of buttered bread on top.
- Heat pan or skillet over medium heat. Grill sandwich on both sides—until golden brown and cheese has melted. Halfway through grilling, cut sandwich in half to ensure the heat gets to the cheese in the center and melts it.