Heat a saucepan until hot, add the mussels, wine and thyme and cover with a tight-fitting lid. Bring to the boil, and cook for 2-3 minutes, or until all the shells are open. Drain, reserve the cooking liquid, remove the mussels from the shells and set aside.
Melt 25g/1oz of the butter in a large pan until then fry the onion and garlic for 2-3 minutes, or until softened.
Add the risotto rice and stir well to coat in the butter. Fry for 1-2 minutes, or until the rice grains start to turn translucent.
Add the wine and cook for 3-4 minutes, or until nearly all of the liquid has evaporated.
Pour in the reserved mussel cooking liquid and cook until evaporated, and then add a ladleful of the chicken stock to the pan and stir well. Cook for 1-2 minutes, or until nearly all of the stock has been absorbed, then add another ladleful of the stock. Repeat the process until the rice is cooked, but still al dente.
Add the reserved mussels, stir in the mascarpone, parmesan and the remaining butter. Season, to taste, with salt and freshly ground black pepper.
Dredge the mackerel in flour and season with salt and freshly ground black pepper. Meanwhile heat the oil in a frying pan until a medium heat.
Shake the mackerel fillets to remove any excess flour, then add them to the pan, skin-side down, and fry for 1-2 minutes, or until crisp.
Carefully turn the mackerel fillets over and fry for a further 1-2 minutes, or until completely cooked through.
For the sauce, heat a saucepan until hot, add the white wine and cook until the volume of liquid has reduced by half.
Add the fish stock and reduce by half again. Pour in the cream and reduce once more until thick enough to coat the back of a spoon. Season to taste with salt and black pepper.
Bring a pan of salted water to the boil and add the celeriac strips.
Cook for 1-2 minutes, or until just wilted and drain. Mix with a little olive oil, and season to taste with salt and freshly ground black pepper.
To serve, place a spoonful of risotto in a bowl and place the mackerel on top. Layer the celeriac strips over the mackerel and finish with a spoonful of cream and some cress.