1. Place the sugar in your food processor.
2. Add the lime zest and process until well combined.
3. Add the butter and process to blend.
4. With the processor running, add the eggs one at a time, followed by the lime juice and the salt.
5. Place a clean bowl and a fine mesh strainer close at hand by the stove.
6. Pour the curd mixture into a small saucepan and cook very slowly over a low heat, stirring constantly, until slightly thickened (about 10 minutes). Do not allow the custard to boil. To check the consistency, lift your stirring spoon from the mixture and swipe a finger across its back; if your finger leaves a clear path on the spoon, the curd is ready to come off the heat.
7. Immediately remove from the heat and pour the curd through a strainer into a medium bowl.
8. Refrigerate for 4–6 hours, until well chilled. Curd keeps in the refrigerator for several weeks.