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.