In a Mixing Glass, muddles the mint with the limoncello. Add the pinch of salt and muddle just enough to mix it in. Add the watermelons to the glass and muddle until smooth (this may take a few minutes). Strain through a medium-fine strainer into a rocks glass. You’ll need to use the back of a spoon to push the solids through to help get the remaining liquid in your glass. Add the champagne and garnish with a mint sprig.
This drink is a great balance of flavors with the addition of salt (just like in baking). The watermelon is present, but not overpowering to make it too sweet. That champagne cuts through to help aid in that as well. And in the background there is a nice herbal-citrus note that finishes well. I was really happy with this, and I hope you can enjoy it too.
UPDATE: instead of using the strainer to get the last bits in, you can mix and muddle your ingredients in a bar shaker and then flip it over into the glass and push solids through the top. Less dishes and mess.