Mango chutney is available in the Asian-food section of most supermarkets.
In a small bowl, whisk reduced-fat mayonnaise with Dijon mustard; spread on bread. On bottom slice, spread mango chutney.
Layer with cheddar cheese, roast beef, and watercress. Slice in half, and wrap in waxed paper. Refrigerate until ready to serve, up to overnight.
