Preheat the oven to gas mark 6 (200°C).
Check the salmon fillets for any bones and remove with tweezers if necessary.
Roll the salmon in the seasoned flour.
Heat the oil in a metal handled frying or sauté pan (one that can transfer in the oven ideally).
Add the salmon, skin side down, reduce the heat and fry the skin slowly for 2 - 3 minutes or until the skin releases from the pan, if you turn it too quickly the skin will stick to the pan.
Once the skin is released, turn the salmon fillets over.
Sprinkle the onion and sage around the fish and put straight into the oven, uncovered.
Roast for 7-9 minutes, or until the fish is cooked, it will have lost its translucency and if you press it is possible to feel the flakes separate.
Lift the fish onto a serving dish to keep warm. Put the pan back on a medium heat (taking care to cover the handle so that it you don’t burn your hands).
Heat the Old Pulteney in a small saucepan and ignite and then pour, flaming, over the onions, taking care not to allow them to burn.
Swirl around to allow the whisky to reduce by half.
Add the stock and reduce again to about half of the original quantity.
Pull the pan off the heat and whisk in the crème fraiche, return to the heat and bring back to the boil and bubble until a creamy consistency.
Season to taste adding a splash of lemon if necessary to round the flavour off.
Heat the butter in a frying pan and season well with salt, pepper, and grated nutmeg.
Once the butter is beginning to brown, add the spinach and cook over a high heat for a few seconds or until it has wilted.
Divide the spinach between two warm serving dishes, arrange the salmon over the top and drizzle with the Old Pulteney sauce.
Serve with mashed root vegetables or new potatoes to complete the dish.
Alternatively, peel the skin away from the salmon, lightly break up the fillets and toss into cooked penne pasta.