First, prepare the rösti – parboil whole potatoes for about 10 minutes, let cool, remove the skins, which should peel off
fairly easily, then coarsely grate the potatoes into a bowl. Season well with sea salt and freshly ground black pepper
and tip into a small frying pan with a little olive oil. Spread out to form a flattened disk and fry until golden brown.
Place a plate on top of the frying pan and invert it, then slide the rösti back in to the pan to cook the other side.
Meanwhile, fry sliced mushrooms in a knob of butter in another pan until they have released most of their moisture and
are starting to brown, then add halved cherry tomatoes. Cook for a minute or so, until the tomatoes start to soften,
then add spinach and cook until just wilted. While these are all cooking away nicely, slash the skins of mackerel fillets
with a sharp knife, season well with sea salt and freshly ground black pepper and fry, skin side down, in a little olive oil
until the skin is crispy, flip over and cook on the other side for another minute or so. Divide the rösti in to two and place
each portion on to hot plates, spoon the veggies beside them and top with the mackerel. A squeeze of lemon juice would be good!