To make the Thai curry paste, peel and roughly chop the garlic, shallots and ginger and place into a food processor. Trim the lemongrass, remove the tough outer leaves and chop into small pieces and add into the food processor. Destalk the chillies, and add them into the food processor along with the cumin and half of the coriander. Blend until finely chopped, add the fish sauce and blend again. (If you do not have a food processor you can use a pestle and mortar)
If your prawns are not peeled peel them and de-vein by slicing along the back of the prawn and remove the black vein. Place on a plate to one side.
Slice the mushrooms and fry over a medium heat for 4 to 5 minutes, or until golden. Transfer to a plate using a slotted spoon.
Reduce the heat to medium-low and add the Thai green paste for 4 to 5 minutes, stirring occasionally. Pour in the coconut milk and 400ml of boiling water, crumble in the stock cube and add the lime leaves. Turn the heat up and bring gently to the boil, then simmer for 10 minutes, or until reduced slightly.
Stir in the tiger prawns and mushrooms, reduce the heat to low and cook for a further 3 minutes, or until the prawns are cooked through, add the mangetout for the final 2 minutes.
Season carefully to taste with sea salt and freshly ground black pepper. Pick, roughly chop and stir through the basil leaves and remaining coriander leaves. Serve with lime wedges and steamed fragrant coconut spiced rice.