Use a vegetable peeler or paring knife to remove the zest from the Meyer lemons (save the fruit for another use). Mix the lemon peel with the mint, sugar, and salt and let sit for 8 hours, stirring occasionally, until the sugar has dissolved into the lemon oil, forming a thick paste. Strain through a
fine-mesh strainer and add the vinegar to the sugar mixture, stirring to combine. Bottle and refrigerate; use within 2 months.
To serve, mix with chilled seltzer water to taste.
