Thai Yellow Chicken Curry With Potatoes and Tomatoes

1 small cooking onion

1 piece galangal or 1 piece ginger , about 3 large garlic cloves

1 teaspoon ground coriander

1 teaspoon ground cumin

1/4 teaspoon fresh nutmeg

3 tablespoons fish sauce

1 teaspoon dried turmeric or 4 cm fresh turmeric, about 4cm long, sliced

1 tablespoon brown sugar

4 kaffir lime leaves , snipped into small pieces with scissors (discard central stem)

1 (400 ml) can coconut milk

1 tablespoon fresh lime juice (or juice from 1/2 lime)

1 tablespoon ketchup

1 kg chicken piece

1 stalk lemongrass or 2 slices lemon rind

2 potatoes , scrubbed and chopped into wedges

1 red capsicum , de-seeded and sliced

1 -2 additional kaffir lime leaf , left whole

1 cup cherry tomatoes

1/2 cup fresh coriander or 1/2 cup fresh basil , torn, to finish the dish