Finely grind the mustard and peppercorns, pour the cider over them, stir well, cover and leave to soak for 4 hours.
Core, peel and finely grate the apple, add to the grains with the apple cider vinegar, rosemary, sugar and salt. Bring to the boil and reduce to the desired consistency over low heat.
Pour the mustard into jars while hot, close and let cool down slowly.
Best for 1 year when stored in a cool, dark place. Store opened glasses in the refrigerator.