*Adapted from Ali Slagle via NYT Cooking
2 tablespoons unsalted butter
1 pound onions, half white and red onions, peeled, halved and thinly sliced
Kosher salt and black pepper
2 teaspoons sherry vinegar
A couple splashes of Worcestershire sauce
4 ounces Gruyère cheese, grated
4 slices bread, cut no wider than 1/2-inch thick
2 tablespoons (or more) of mayonnaise
- In a large skillet over medium-high heat, melt 2 tablespoons butter. Add the onions and season with salt and pepper. Cover and cook, stirring 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 to 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 that are stuck to the bottom of the skillet. Once the onions are done, deglaze the skillet with vinegar and worcestershire 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. Put back on the stove over medium-low heat.
- 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.
- Spread mayonnaise on exposed slices of bread in a thin layer, covering edge to edge (you want a nice even layer across the bread for perfect toasting). Add the sandwiches to the skillet and cook until the bottoms turn golden brown, 3 to 4 minutes, reducing the heat to prevent toast from darkening too quickly, if needed. 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.