In a bowl stir together the softened butter, miso and mirin, then set aside.
On a hob, heat a lidded, deep oven-proof frying pan and toast the sesame seeds until golden, then set aside.
Heat the oven to 200C (180C fan)/390F/gas 6.
Add the oil to the pan and fry the shallots over a medium heat for 10 minutes or until softened.
Add the garlic and ginger and cook for 4 minutes.
Stir in the cavolo nero and cook for 5 minutes.
Add the carrots, rice and water, bring to a simmer then pop a lid on and cook over a low heat for 10 minutes.
Stir everything together, adding the soy.
Pat the cod dry with kitchen paper then spread with the miso butter.
Nestle the cod in the rice, cover, then slide into the oven for 8 minutes.
Remove and then leave to sit with the lid on for another 10 minutes.
Scatter over the toasted sesame seeds and shredded spring onion, serve with wedges of lime to squeeze over.
