For best results, ensure the duck skin is crispy when adding it to the sandwich.
The blackberry mayonnaise is key to the flavor profile; do not skip it. Adjust sweetness by varying the amount of jelly.
If using leftover cold duck, you can warm it slightly in a pan to render some fat and crisp the skin again before assembling.
Bean sprouts add a nice "hairy" texture contrast, but can be omitted if preferred.