Hot sandwich with cheese and caramelized onions

Ingredients:

• 12 slices rustic Italian bread (each about 1 cm thick)
• 220 g thin slices of Gruyere
• 2 tbsp. l. rapeseed oil (canola)
• 70 g unsalted butter, softened
• 2 large onions, thinly sliced
• 2 tsp sherry vinegar
• 1 teaspoon of sugar

Preparation:

1. In a large skillet over medium heat, heat the canola oil with 1 tablespoon of butter. Once the butter has melted, add the onion and cook, stirring occasionally, until dark brown and caramelized, about 25 minutes. Add the vinegar, sugar, and 2 tablespoons of water and stir, scraping up any browned bits from the bottom of the pan. Continue cooking the onion until all the liquid has evaporated. Transfer the onion to a bowl and set aside.
2. Butter 1 side of each slice of bread. Place 6 slices buttered side down on a work surface. Top with 2 slices of Gruyère, a spoonful of caramelized onions, and 2 more slices of Gruyère. Top with a slice of bread, buttered side up.
3. Heat a cast-iron skillet over medium heat. Place the sandwiches in the skillet and cook until the cheese begins to melt and the bread is golden brown, 2-4 minutes. Flip and continue cooking until the cheese is completely melted and the bread is golden brown on the other side, 2-4 minutes more. Cut the sandwiches in half and serve immediately.