Cut the lemongrass down its length and then into thirds so you get 6 pieces. Put those in a cocktail shaker along with the cilantro, the lemon juice, and the watermelon cubes. The cubes do not need to be exactly uniform in size, but you want to keep them somewhat smallish so you can muddle them more easily and consistently. Proceed to muddle, and realize this might take some time. Yes, you could put the watermelon in the blender, but then you’d have to clean your blender and pick out all the seeds before blending, too. Muddling saves you all that. Just keep at it, acting like you have to “pop” each chunk. It should be very much a liquid when you’re done. Add the tequila and St-Germain. Add ice and shake well—and again, do this a bit longer than you usually might to integrate all that content— until well chilled.
At this point you can choose which of 2 cocktails to make. If you want something up and elegant, strain (you must strain to get all the cilantro, lemongrass and watermelon pulp and seed out) into 2 chilled up glasses, give each the smallest pinch of sea salt (it really helps the flavors sing, so don’t skip this step) and stick a melon edge on each rim. That’s The ’Rena Neat.
If you want a longer-lasting drink for a hot summer afternoon, strain an equal amount into 2 half-pint beer glasses. Gently pour the pilsner to fill; it’s very easy to have the drink foam up, so go slowly. (Note: You won’t use a whole bottle for 2 cocktails, so be prepared to make more or drink some beer later.) Give the mildest of stirs. Top each with the smallest pinch of salt, and stick a melon edge on each rim. That’s The Micheltorena.