Chicken & spinach curry

1 tablespoon vegetable oil

4 boneless, skinless, chicken breasts about 125g (4oz) each, halved lengthways

1 onion, sliced

2 garlic cloves, chopped

1 green chilli, chopped

4 cardamom pods, lightly crushed

1 teaspoon cumin seeds

1 teaspoon dried chilli flakes

1 teaspoon ground ginger

1 teaspoon ground turmeric

250 g (8 oz) baby spinach leaves

300 g (10 oz) tomatoes, chopped

150 ml (¼ pint) low-fat Greek yogurt

2 tablespoons chopped coriander, plus extra sprigs to garnish