This sandwich was devised using La Brea Country White Sourdough Oval Country White Sourdough Oval bread - 8 slices Extra virgin olive oil - 1/3 cup Tablespoons white wine vinegar - 2-3 Tablespoons Maldon sea salt - 2 Tablespoon freshly cracked black pepper - 1 Medium yellow onions, sliced 1/8-inch thick - 2 Wholegrain mustard - 1/4 cup Swiss cheese, sliced 1/16-inch thick - 8oz To prepare the marinated onions Combine the oil, vinegar, salt and pepper in a bowl. Add the onions, toss to coat them, and marinate for 15-20 minutes at room temperature. Season with more vinegar, salt, and pepper To assemble the sandwiches 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 using a panini press. Cut each sandwich in half diagonally.