Gruyère and caramelized onions are the perfect duo in this satisfying sandwich. Have it as a snack or make it a meal with some soup on the side. This is also tasty made with Swiss, Fontina, or Cheddar cheeses.*
*Note: This could affect the PointsPlus value.
Coat a large nonstick skillet with cooking spray and set over medium heat. Add the onion; reduce heat to medium-low and cook, stirring often, until tender and golden brown, about 8-10 minutes. Set aside.
Lay 2 slices of bread on a work surface and spread 1 teaspoon mustard on each slice. Top each slice with half of the onions and cheese and cover each slice with a second piece of bread.
Wipe the skillet clean and set over medium-low heat. Coat the sandwiches with cooking spray. Cook until the bottoms are browned, about 2 minutes. Carefully flip the sandwiches over and cook until browned on the other side and the cheese is melted, about 2 more minutes. Slice each sandwich in half and serve.