Cut off the watermelon rind, then chop the flesh into chunks, picking out any seeds. Roughly chop the ginger and place in a large sealable freezer bag with the watermelon chunks. Finely grate in the lime zest, squeeze in all the juice, then freeze flat for at least 8 hours, or until super-hard.
When ready to serve, pick and reserve the baby mint leaves, then pick the rest into a food processor. Tip in the frozen watermelon mixture and blitz to a pink snow (in batches, if you need to). Serve 2 heaped tablespoons of granita per person, with 1 tablespoon of yoghurt, a drizzle of ginger syrup from the jar and a few baby mint leaves. It’s nice to use chilled plates, bowls or glasses here.