In a small saucepan combine the brown sugar, apple juice, cinnamon, nutmeg, and salt. Stir to combine and dissolve the sugar.
Bring the mixture to a light simmer over medium low heat, and simmer for 10 minutes, stirring occasionally.
The mixture should thicken slightly to be a slightly thicker syrup texture rather than a thin juice texture. If the syrup thickens like a sauce, that means your heat is too high.
Remove the saucepan from the heat and strain using a fine mesh strainer, into a mason jar or syrup bottle.
Allow the syrup to cool at room temperature for at least one hour.
Add a lid and store the syrup in an airtight container, and in the fridge for up to 3 weeks.