Grind the spice paste ingredients in a mortar and pestle into a smooth consistency.
Wash the lamb rump pieces in cold water and pat them dry.
Coat the lamb in the spice paste and marinate in the fridge for at least one hour.
Heat oil in a heavy lidded pan and fry the lamb for 3 minutes until browned on all sides.
Cover the pan and cook over medium heat for 5 minutes.
Deglaze the marinade bowl with 75ml of water and add to the lamb; cook covered for 5 minutes.
Add the stout and sugar mixture, bring to a boil, then simmer for 8 minutes until thickened.
Place freshly grated coconut in a bowl and pour boiling water over it.
Steep the coconut for 20 minutes then squeeze out the liquid and discard the solids.
Boil the coconut liquid with beans, spring onion, chilli, garlic, thyme, allspice, butter, and onion.
Simmer the vegetable mixture for 15 minutes until tender.
Add rinsed rice to the pan, ensuring liquid covers it by 1.25cm, and season with salt and pepper.
Cover and cook on the lowest heat for 20 minutes until the liquid is absorbed.
Discard the chilli, serve lamb over rice, and garnish with mint.