Step 1

Zest the lemons, limes, along with the grapefruit and orange into a medium bowl. Remove the fibrous outer layers and tops and bottoms from the lemongrass, leaving the tender center of the stalk. Finely chop the center of the stalk and add to the bowl. Finely slice the serrano chili, and add it, seeds and all, to the bowl.

Step 2

Transfer the ingredients to a mortar, add the salt, and pound with a pestle until everything combines into a rough paste (alternatively, blend in a food processor or blender into a fine paste). Juice one half of a lemon and one half of a lime and add the juice to the paste and stir. If using, add the lemon, orange, and lime oils, and stir.

Step 3

Transfer to a jar and refrigerate for at least two days to allow the flavors to mingle before using.

Step 4

Will keep in the refrigerator for about a month.