Place the annatto, peppercorns, cumin, coriander, garlic, cloves, pimento and oregano into a spice grinder. Grind until powdered.
Transfer to a bowl. Mix in the salt and nutmeg, then add the vinegar and water. Combine well, press into a plastic container or jar and store in the fridge. Alternatively, freeze it and you can break off what you need when the time comes!