- 1/3 Cup Olive Oil, Extra Virgin
- 2 ½ Tbsp. White Wine Vinegar
- 2 Tbsp. Kosher Salt
- 1 Tbsp. Freshly Cracked Black Pepper
- 2 Medium Yellow Onions, Sliced into 1/8-Inch-Thick Slices
- ¼ Cup Whole Grain Mustard
- 8 oz. Gruyere cheese, sliced into 24-32 1/16 inch-thick slices
- Country White Sourdough Loaf
- In a medium bowl, combine the oil, vinegar, salt, and pepper. Add the onions, toss to coat them, and allow to marinate for 15-20 minutes at room temperature. Season them with more vinegar, salt, and pepper, to taste.
- Set half of the slices of bread buttered side down. Spread an even layer of mustard over the bread and cover with half the cheese slices, folding them back in toward the middle if they extend past the edges of the bread. Scatter the marinated onions on top and place the remaining cheese slices over the onions. Put the top slices of bread over the cheese, buttered side up. Grill the sandwiches in a moderately hot skillet until the bread is golden brown and the cheese has melted. Cut each sandwich in half on the diagonal.