Prepare the meringue as directed in this recipe card. In a blender, combine the condensed milk, lemon zest, and salt. Blend briefly to release the lemon oils.
Add the lemon juice and water. Blend until smooth.
Add 3 cups of ice and blend on high until thick and slushy, adding the remaining cup if needed. Taste and adjust, add a splash of water if it’s too thick, more ice if you want it thicker, or adjust the sweetness to your liking.
Pour into chilled glasses. Top with meringue and lightly torch it. Serve immediately.