To a 32 oz. deli container, add the watermelon, mint leaves, quartered lemon, honey, and a pinch of flaky sea salt. Muddle until the watermelon is juicy and the honey has dissolved. Add the elderflower liqueur, vodka, and rosé.
Fill the deli container with ice and top with a splash of sparkling water, if desired.
Garnish with a fresh mint sprig and a watermelon wedge, and serve straight from the deli container with a straw.