Cut the fish into slices about 4cm thick.

Slice the tomatoes finely. You don’t need to save the juice, so leave whatever runs out on to the chopping board, as the slices shouldn’t be too wet. Scrub then slice the potatoes finely, using a mandolin if you have one. It’s important that the slices are really wafer thin or the potatoes won’t cook through in the time given and you’ll have to cook the gratin for longer, which means the fish will be overcooked.

Layer up half the potatoes and half the tomatoes in a metal gratin dish about 30cm in diameter, or an oval shaped dish or casserole measuring around 36 x 23cm, with a capacity of at least 2 litres, seasoning and sprinkling on the thyme as you do so.

Heat the cream and garlic in a saucepan to boiling, then strain it into a bowl and stir in the mustard, to taste. Pour half of the cream mixture over the vegetables.

Lay the fish on the cream soaked vegetables, then add the rest of the vegetables and pour over the remaining hot cream. Sprinkle both cheeses on top.