1. Bring 2 quarts of water to a boil over high heat, add 1 teaspoon of the baking soda, stir to combine, and then add the limes. Cook at a soft simmer until slightly tender, 10 to 15 minutes. Remove the limes from the water with a slotted spoon and let cool.
2. Make a small incision in the top of each lime with a sharp paring knife and carefully scrape out the flesh, making sure you don’t tear the rind; discard the filling. Return the intact rinds to the pot, add cold water to cover, and stir in the remaining 1 teaspoon baking soda. Bring to a boil, strain, and repeat this process (without any more baking soda) 3 more times to remove the bitterness from the limes.
3. Return the limes to the pot, add cold water to cover, then stir in 1 1/2 cups of the sugar and a few drops of food coloring. Cook over medium-low heat, stirring, until the syrup has thickened to the consistency of corn or maple syrup, 15 to 20 minutes. Remove from the heat, let cool completely in the syrup, then transfer the limes to a wire rack and let dry.
4. Combine the coconut, the remaining 1 cup sugar, and the 1/2 cup water in a saucepan and cook over medium heat, stirring, until the coconut is soft, almost translucent, and thick. Let cool until it is safe to handle.
5. Fill the limes with the coconut mixture and let cool completely. Eat by biting into the lime. Store in an airtight container lined with parchment paper in a cool, dry place for up to 2 months.
Andrea Reusing is the creator of the restaurant Lantern in Chapel Hill, N.C., and author of the book Cooking in the Moment: A Year of Seasonal Recipes. In this installment of The Key 3, she shares with Lynne Rossetto Kasper the techniques behind three of her favorite recipes: Turnip Soup, Overnight Braised Short Ribs and Tomato Salad.