Creamy cod and potato gratin

Cover the peeled, quartered russets with generously salted water; bring to a boil, then hold at a gentle simmer until tender, when a knife slides in with virtually no effort.

Cut the cod across the fillet into roughly two-inch pieces on a cutting board.

Drain the potatoes; add a little cayenne, a big pinch of salt and about two tablespoons butter per potato; mash as smooth as possible and taste for salt. No milk or cream.

Spread mashed potato in lightly buttered individual baking dishes set on a pan, smoothing it and making a well in the centre.

Melt butter over medium-high; saute the shallots with a pinch of salt 3 to 4 minutes, until just turning golden but not browned.

Add the cream, lemon zest, another shake of cayenne and a pinch of salt; bring to a boil on medium-high, watching that it does not boil over.

Add the cod, toss and baste; simmer about a minute, flip, and simmer another minute.

Turn off the heat and stir in the lemon juice for about 30 seconds; the fish is not cooked through.

Divide the fish over the potatoes, mixing big and small pieces; spoon over about half the sauce.

Return the remaining sauce to medium-high and boil until it thickens rapidly, scraping any cream caramelizing on the sides back in.

Off the heat stir in the tarragon; spoon the thick sauce over the fish.

Dust very lightly with parmigiano reggiano.

Bake in the centre of a 450 degree oven 10 to 15 minutes until the fish is cooked through and it is browned and bubbly.

Rest about five minutes before serving.