- 3 tablespoons extra virgin olive oil
- 1 tablespoon champagne or white wine vinegar
- 1/2 teaspoon coarse sea salt
- 1 1/2 teaspoons freshly cracked black pepper
- 1 yellow onion, halved and sliced 1/8-inch-thick
- 8 slices sourdough, Mille Grane or Pain du George bread
- 2 tablespoons melted butter or olive oil, for brushing
- 1/4 cup whole-grain mustard
- 8 ounces Gruyère cheese, thinly sliced
Preheat a large skillet to medium heat.
In a bowl, combine oil, vinegar, salt and pepper. Add onions, toss to coat. Marinate for 15 to 20 minutes at room temperature.
Lightly butter one side of each slice of bread (or brush with olive oil). Place half of the slices buttered side down, spread an even layer of mustard over bread and cover with half the cheese slices. Scatter marinated onions on top and place remaining cheese slices over onions. Put the top slices of bread over cheese, buttered side up.
Grill sandwiches in a moderately hot skillet for 4 to 5 minutes until bread is golden brown and cheese has melted, turning sandwiches over halfway through grilling. Cut each sandwich in half on the diagonal and serve immediately.