To make the coulis: In a medium saucepan, combine the cranberries, sugar, water, and lime zest. Stir over medium heat until the sugar dissolves. Bring to a boil and then reduce heat to simmer the mixture gently for about 10 minutes. The cranberries will burst and it'll thicken slightly. Remove from the heat and cool slightly before puréeing in a blender or food processor. Strain the puréed mixture back into the saucepan, discarding any strained-out solids. Stir in the lime juice. Bring the mixture back to a boil; it will thicken slightly. Remove from the heat and cool to room temperature. Refrigerate until ready to use, at least 1 hour. You won’t need all of the coulis for the meringues; it can be stored, covered, in the refrigerator for up to four weeks.

To make the meringues: Preheat the oven to 200°F. Lightly grease (or line with parchment) two large baking sheets. In a large bowl, beat the egg whites until foamy, then add the salt and cream of tartar. Beat until soft peaks form, then gradually add the sugar, continuing to beat until the mixture is stiff and glossy. Drop heaping tablespoonfuls of the mixture onto the prepared pans; a level tablespoon cookie scoop works well here. Portion out about ¼ cup (about 75g) of the coulis. Place ¼ teaspoon coulis in the center of each meringue. It works well to dip a ¼ teaspoon measuring spoon into the coulis to fill, then use the coulis-filled measuring spoon to make a dimple in the middle of the scooped meringue. Rotate the measuring spoon to transfer the coulis to the center of the meringue, while pulling the measuring spoon up and away from the sheet pan. Repeat with the remaining meringues. Using the handle of the measuring spoon, the tip of a butter knife, a skewer, or a toothpick, swirl the coulis into the meringue.

To bake the meringues: Bake the meringues for 4 hours. Turn off the oven and leave them inside to cool and dry for at least 1 hour (or as long as overnight if the weather is humid); they should be completely dry and crispy.