You can prepare the chicken salad up to a day ahead and refrigerate; assemble the sandwiches just before serving.
24 (1-ounce) slices pumpernickel bread
2 1/2 tablespoons finely chopped onion
1 1/2 tablespoons dark brown sugar
1 1/2 tablespoons cider vinegar, divided
2 cups chopped roasted skinless, boneless chicken breast
3 tablespoons reduced-fat mayonnaise
1/2 teaspoon curry powder
1/8 teaspoon salt
How to Make It
Trim crusts from bread slices; cut slices into triangles.
Heat a small nonstick skillet over medium-high heat. Coat pan with cooking spray. Add onion to pan; sauté 5 minutes. Combine sugar and 1 1/2 teaspoons vinegar, stirring with a whisk until sugar dissolves. Add sugar mixture to pan, scraping pan to loosed browned bits. Remove from heat; cool to room temperature.
Combine onion mixture, remaining 1 tablespoon vinegar, chicken, and remaining ingredients. Place about 2 tablespoons chicken mixture on each of 12 bread slices. Top with remaining bread slices.