Heat half the butter in a small sauce pan and gently soften the chopped shallot until it is translucent.

Add the champagne and tarragon and bring to a gentle simmer.

Reduce the mixture by 50% - this will take 15 minutes or so

Meanwhile trim the asparagus, if necessary peeling the woody parts of the stems

Bring a pan of salted water to the boil and add the asparagus. Cook for 5-7 minutes until tender then drain and keep warm

Dry the halibut fillets with kitchen paper, then season with salt and pepper

Heat the oil in a skillet until just smoking. Add the halibut, skin side down and reduce the heat to medium

Cook for 3 to 4 minutes until the halibut is just opaque through and the skin is crispy and golden. Leave the fish in the pan. cover and put to one side while you finish the sauce

Remove the tarragon from the sauce and add the cream. Warm through gently.

Whisk with an electric stick blender till all the shallots are blended and the sauce is slightly foaming. Taste and adjust the seasoning with salt and pepper. Add lemon juice and whisk again.

Toss the drained asparagus in the remaining butter

Put a pool of sauce on each plate, then lay out the asparagus spears. Put the fish onto of the asparagus and garnish with cayenne pepper and a sprig of tarragon or a slice of lemon.

Serve with new potatoes