Preheat broiler.

Combine apricot preserves and Dijon mustard in a small bowl and mix well.

Spread 2 teaspoons of the apricot-Dijon mixture on one side of each bread slice.

Place bread slices, mustard-side up, on a baking sheet.

Heat a large skillet over medium-high heat.

Coat the skillet with cooking spray.

Add thinly sliced apple and chopped onion to the skillet.

Sauté the apple and onion for 3 minutes, or until tender.

Add fresh spinach leaves to the skillet.

Cook for 1 minute, or until the spinach wilts.

Layer each of the 4 bread slices with 1/4 cup of the spinach-apple mixture.

Top the spinach-apple mixture with 1/2 cup of shredded rotisserie cooked chicken breasts.

Place one slice of Muenster cheese on top of the chicken on each bread slice.

Broil for 1 1/2 minutes, or until the cheese is bubbly and melted.

Remove from heat and serve immediately.