Mix together half the olive oil, the walnut oil, lemon zest and juice, and sherry in a shallow dish. Place the salmon fillets in the dish, turn to coat with the dressing, then cover and leave to marinate in the refrigerator overnight.
Place the zucchini ribbons in a large bowl, add the remaining oil and toss together. Set aside. Arrange the salad leaves on four serving plates, top with the grapefruit segments, then scatter the walnuts over the top. Set aside.
Heat a cast-iron, ridged grill pan over a high heat and cook the zucchini ribbons (in batches, if necessary) for 4-5 minutes, turning often, to produce charred stripes on the surface. Remove from the pan, set aside and keep warm.
Remove the salmon from the marinade, reserving the marinade. Reduce the heat to medium and put the salmon, skin-side down first, on the pan and cook for 4-5 minutes on each side until cooked through and firm to the touch.
Spoon some zucchini ribbons onto each portion of salad and top each with a piece of salmon. Pour the marinade into the grill pan and simmer on a high heat for 1-2 minutes until reduced slightly. Pour over the salmon and serve.
Cook's tips: To make zucchini ribbons, use a vegetable peeler to peel off thin ribbons, drawing the peeler along the length of the zucchini until you are left with only a slim centre core, which can be discarded. If you don't have a grill pan, cook both the zucchini ribbons and the salmon under a preheated broiler.
Each serving: 31 g protein, 32 g fat of which 5 g saturates, 5 g carbohydrate, 2.5 g fibre, 452 Calories