Put the orzo into a medium-sized ball and cover with boiling water. Set aside to soak for 15 minutes, then drain and rinse.
Heat the butter and oil in a large, shallow sauté pan over a medium-low heat. Add the onion, pepper, the whole Scotch bonnet and the salt. Cook gently for about 8 minutes, or until soft but not coloured.
Add the tomatoes, tomato purée and saffron and stir-fry for 2 minutes, then stir in the coconut milk and simmer for 3 minutes.
Add the orzo, then spread the mussels out on top. Cover with a lid, turn the heat to the lowest setting and cook for 5 minutes or until the mussels have all opened.
Discard the whole chilli, you can give it a squeeze into the sauce first if you like heat. Taste the sauce and add some chilli paste if you like.
Squeeze plenty of lemon juice over the mussels, then mix all the ingredients for the herb oil together and spoon over the top.