Make the simple syrup. Add the monk fruit and 1½ cups of water to a small pot. Cook over medium heat until the sweetener is dissolved, about 5 minutes. Turn heat off under the pot and let cool to room temperature or add some ice if you want to cool it quickly. If the syrup begins to crystalize, just reheat it.
Meanwhile, juice 1½ cups of lemon juice in a bowl or pitcher and add 3