Make these onions at least 1-2 days ahead if you want them to develop this deep pink colour.
1 small red onion, peeled and halved
3 tablespoons lemon juice
1 teaspoon sea salt
½ teaspoon cumin seeds
Slice the onions very thinly (a mandolin is perfect for this) and place in a heatproof glass or ceramic bowl.
Pour over enough boiling water to cover and leave for 20 seconds. Drain well and return to the bowl. Add the lemon juice, salt and cumin, and toss to coat. Cover and refrigerate for at least 1 hour, turning occasionally. Drain and place in a bowl to serve. The onions can be made 3 days ahead.