Put the watermelon pieces in a freezerproof container and freeze overnight until solid. The next day, prepare the garnish for the rim of each glass. Combine the sea salt flakes, sugar, chilli flakes and lime zest in a small bowl. Set aside.

When you want to serve, put the frozen watermelon, tequila, triple sec, lime juice and honey in a food processor or blender and blitz until smooth. Run a lime wedge around the rims of four glasses, then dip in the garnish mix. Pour the drink into the glasses. Serve immediately.