For the salmon: Pat the salmon fillets dry with kitchen paper. Season the flesh side lightly with salt and pepper. In a large non-stick frying pan, on a medium heat, heat the butter until it is foaming and add the salmon fillets to the pan, skin side down. Very fresh fish will arc upwards at this point, so using a spatula or a flat utensil, press the fillets down for a few seconds so they keep contact with the pan and stay flat against it. Cook the salmon fillets for 7-8 minutes depending on their thickness, until the skin is crisp (*1). Turn the fillets over onto the flesh side and cook for a further 1 minute. Remove the salmon fillets from the pan and leave to rest for 1 minute on a warm plate. Add the water and a squeeze of lemon juice to deglaze the pan and reserve these cooking juices while you prepare the spinach and watercress next.
For the spinach, watercress and Saint Agur sauce: In a large saucepan, on a medium heat, sweat and sweeten the shallots in the butter for 3-4 minutes without colouring. Add the watercress, spinach, whipping cream, Saint Agur blue cheese, salt and pepper. Cover and cook on full heat for 1 minute, stirring occasionally. Taste and adjust the seasoning if necessary.
To serve: Spoon a mound of the spinach, watercress and Saint Agur sauce into the center of each plate. Place a fillet of pan-fried salmon on top, then spoon the reserved cooking juices over the salmon and serve.
TIP (*1) You should hear just a gentle sizzle at this point and see the browning and crisping of the salmon skin. But be careful – Too high a heat will burn the skin, overcook the salmon and burn the butter so it becomes bitter.
As an alternative to the spinach and watercress, you could also use blanched parsley, spring greens or sorrel should you wish.