INGREDIENTS

• 3 tablespoons unsalted room temperature butter, divided
• 1 vidalia onion, sliced 1/8" thick on a mandoline
• Sea salt
• 1 dried bay leaf
• 1 1/2 tablespoons cognac
• 1/4 cup beef stock
• Freshly cracked black pepper
• 8 3/4-inch slices of sourdough bread, about 1 8" round loaf (see Note #1)
• 8 ounces gruyère, coarsely grated
• 8 ounces Italian fontina, coarsely grated (see Note #2)

PROCEDURE

1. Melt one tablespoon of butter in an 8.5-inch sauté pan over medium heat. 
2. Add the thinly sliced onions, salt, and bay leaf. Cook 8 minutes, stirring often with a wooden spoon, until the onions are soft and golden, but not charred. If the onions begin to brown too quickly, add 2 tablespoons of water, and continue cooking.
3. Add the cognac off the heat, and allow to reduce 2 minutes over low heat.
4. Add the beef stock and cook on low heat for 3 minutes, until the onions are soft and jammy, and the pan is dry. Remove the bay leaf and discard.
5. Meanwhile, lightly butter one side of each slice of bread with the 2 remaining tablespoons of butter. Divide the cheese between all the unbuttered sides of bread, and scatter the onion mixture evenly on half of those slices. Then top each piece of onion and cheese bread with a slice of just cheese bread, so the butter is on the outside of the sandwich, and the onions are sandwiched between two layers of cheese.
6. Place in an 8.5-inch sauté pan on medium-low heat, place one sandwich at a time on the hot pan. Toast 2 minutes on the first side, lower the heat to low, and toast 4 minutes on the second, or until the outside of the sandwich is golden and crisp, and the cheese is melted. Cut in half, and serve immediately, with Dijon mustard and a few cornichons on the side.

NOTES

1. You could also use a rustic French farmhouse round loaf. 
2. Feel free to use either a mix of gruyère and fontina, or just 16 ounces gruyère.