For the salmon

• 4 salmon fillets (about 150 g each, skin on or off)
• 15 ml olive oil or neutral oil
• Salt and freshly ground black pepper, to taste

For the Sandefjordsmør

• Juice of 2 lemons (about 90 ml)
• 120 ml double cream
• 70 g cold unsalted butter, cut into small cubes
• Salt, to taste
• Cayenne pepper, to taste
• 10 g fresh flat-leaf parsley, finely chopped

Cook the salmon

1. Pat the salmon fillets dry and season lightly with salt and black pepper.
2. Heat the oil in a large frying pan over medium heat.
3. Place the salmon in the pan, skin-side down if using skin-on fillets and cook for 4–5 minutes until the flesh is mostly opaque and the skin is crisp.
4. Turn and cook for a further 1–2 minutes, until just cooked through. Remove from the pan and keep warm.

Make the sauce

1. Pour the lemon juice into a small saucepan and place over medium heat.
2. Simmer gently until the juice reduces to just under 15 ml and is almost evaporated, about 4–5 minutes. Take care not to let it burn.
3. Quickly add the double cream and whisk to combine.
4. Continue cooking for 5–6 minutes, stirring occasionally, until the cream reduces and is thick enough to coat the back of a spoon.
5. Reduce the heat to low.
6. Begin whisking in the cold butter a few cubes at a time, allowing each addition to melt and emulsify before adding more.
7. Continue until all the butter has been incorporated and the sauce is smooth and glossy, about 8–10 minutes.
8. Season with salt and a pinch of cayenne pepper, then whisk in the chopped parsley.

Serve

Spoon the warm sauce generously over the salmon and serve immediately.