Preheat the oven to Gas mark 4, 180°C (350°F).
Lightly butter a 20 x 30cm baking dish.
To prepare the potatoes: Melt the butter in a large frying pan and cook the onion and thyme on a low to medium heat for 8-10 minutes, until soft.
Place half the sliced potatoes in a layer on the bottom of the baking dish, season with salt and black pepper, and then follow with a layer of the onion mixture. Season, then add the rest of the potatoes. Pour over the hot stock and bake in the oven for 40 minutes.
To cook the monkfish: In a small bowl, mix together the olive oil and nduja paste. Take the potatoes out of the oven and place the monkfish fillets on top, season well with salt and pepper then spoon the paste over the fish. Sprinkle over the panko crumbs. Place in the oven and bake for 15 minutes or until the fish is piping hot and cooked through.
To serve: Divide the monkfish and potatoes between four serving plates and serve with some pak choi wedges which have been sautéed with a little chopped garlic.