From the kitchen of Ali Slagle at https://cooking.nytimes.com/recipes/1020107-french-onion-grilled-cheese
Course Main Course
Cuisine American
Keyword Cheese, Sandwich
Ingredients
4tbspunsalted butter
1lbyellow or Vidalia onionspeeled, halved and thinly sliced
2tspsherry, red-wine or white-wine vinegar(optional)
4oz.Gruyere cheese, grated
4slicesbread, cut no wider than 1/2 inch thick
Salt and Pepper
Instructions
In a large skillet over medium-high heat, melt 2 tbsp. butter. Add the onions and season with salt and pepper. Cover and Cook, sitting once or twice, until the onions are softened, 3 to 5 minutes. Uncover, reduce the heat to medium-low, and cook, stirring occasionally, until deep golden brown, 20-25 minutes. If the onions look dry or like they might burn, add a few tablespoons of water at a time, scraping up any browned bits tar are stuck to the bottom of the skillet. If desired, once the onions are done, deglaze the skillet with vinegar and cook until the liquid has evaporated, about 1 minute. Transfer the onions to a medium bowl and season to taste with salt and pepper. Wipe out the skillet, or wash it, if necessary
Add the cheese to the onions and stir to combine. Put down two slices of bread, and scoop half of the cheese-onion mixture onto each one. Top with the remaining slices of bread, and press down gently.
In the skillet, milt 1 tbsp. butter over medium heat. Add the sandwiches and cook until the bottoms turn golden brown, 3 to 4 minutes, reducing the heat to prevent toast from darkening too quickly, if needed. Add the remaining butter, flip the sandwiches, press down and cook until the cheese has fully melted and the bottoms turn golden brown, 3 to 4 minutes. Serve immediately.