Grilled gruyere and caramelized onion sandwiches

Coat a large nonstick skillet with cooking spray; heat over medium heat. Add onions; reduce heat to medium-low and cook, stirring often, until tender and golden brown, about 8 to 10 minutes. Remove onions to a plate.

Lay 2 slices of bread on a work surface; spread each with 1 teaspoon mustard. Top each with half of the cheese and onions; cover each with a second piece of bread.

Wipe skillet clean; heat over medium-low heat. Coat sandwiches with cooking spray. Cook until bottoms are browned, about 2 minutes. Carefully flip sandwiches over and cook until browned on other side and cheese is melted, about 2 minutes more. Slice each sandwich in half and serve. Yields 1 sandwich per serving.