In a blender purée chunks of watermelon and then pass through a fine sieve into a pitcher (should make 500ml), discarding the seeds and fibre.

In a cocktail shaker 1/3 filled with ice, pour in 40ml tequila, 20ml Cointreau, the juice of ½ lime, 50ml watermelon purée, and a squeeze of agave syrup.

Shake well and strain into a chilled cocktail glass.