Preheat: Panini grill to high setting or skillet to moderate heat
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.
Lightly butter one side of each slice of bread (or brush with olive oil). Place half of the slices buttered side down and 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 or Panini grill for 4-5 minutes until the bread is golden brown and the cheese has melted. If working in a skillet, turn sandwiches over halfway through grilling. Cut each sandwich in half on the diagonal and serve.
Recipe courtesy of Nancy Silverton
Serves: 4