In a food processor, whizz together the shallots, chillies, garlic, ginger, lemongrass, garam masala and 2 tbsp water.
Heat the oil in a pan and gently cook the paste for 2-3 minutes until fragrant. Pour in the coconut milk and 100ml water, then add the tamarind paste. Simmer gently for 15 minutes, adding in the green beans and pepper for the last 10 minutes. Stir through the steak for the last 2-3 minutes of cooking time.
Meanwhile, put the rice in a pan with 500ml cold water. Bring to the boil, then turn down and simmer for 12 minutes, covered.
Stir the fish sauce, lime juice and coriander through the curry and serve with the rice. Garnish with the desiccated coconut and remaining coriander leaves and chilli.