Choose a baking dish or other shallow oven-proof dish suitable for bringing to table, which is just large enough to take the trout in a single layer when they are arranged head to tail.
Thoroughly dry the fish with plenty of kitchen paper.
Make a frying pan very hot, barely smear it with fat and fry the fish briefly, one or two at a time, just long enough to brown their skins well on both sides.
Transfer the fish to the ovenproof dish, arranging them neatly.
Cover and store in a cool place.
Chop the herbs and mix them gently in a small saucepan.
Measure the yoghurt and cream and pour them over the herbs.
Add plenty of pepper and some salt and stir to mix well.
Cover and leave to infuse.
Everything up to this stage can be done in advance, but be sure to bring both the pan of sauce and the dish of trout back to room temperature about 1 hour before they are to be put back into the oven for cooking. (Foods that are transferred straight from the fridge to cooker take a very long time to heat through properly. ) About 1/2 hour before you plan to serve the trout, bring the yoghurt-and- herb mixture very slowly to simmering point, stirring all the while.
Pour the aromatic mixture over the fish, cover the dish and bake at 350 F (180 C) gas mark 4 for 20-25 minutes until the fish are piping hot and cooked through. (To check a trout for readiness, insert the tip of a knife into its flesh just behind the head. ) Serve with steamed new potatoes and other young summer vegetables such as carrots and peas.