A ketchup particularly good with shrimp and white fish.
Thinly slice lemons, and rub them in the salt.
Place the lemons, shallots, garlic and the spices into a jar, reserving a bit of the mace and ginger for later.
Add the horseradish.
Bring the vinegar to a boil, stir in the reserved mace and ginger, boil rapidly 5 minutes.
Pour over the salted lemons.
Cover lightly and allow to cool.
Close jar tightly and refrigerate for 6 months.