Preheat your oven to full whack. Get yourself a large roasting tray that your whole salmon will fit inside – you’ll probably need to lay the fish diagonally across the tray, and it won’t matter if the head and tail drape over a little. Lay your sliced potatoes over the base of the tray and season well with salt and pepper. Scatter your baby fennel over the top and give it all a generous drizzle of olive oil.
Now get on with your fish. Pick half the leaves from each of the herb bunches and put them on a chopping board, keeping the remainder to one side. Grate the lemon zest over the herbs then roughly chop everything together. Scrape this mixture into a bowl and wipe the board down with a piece of kitchen paper.
Transfer your salmon to the board, then wipe it inside and out with a fresh piece of kitchen paper. Make sure there are no scales on the silvery salmon skin – the fishmonger should have taken all of these off for you. If there are any left on there, scrape them with a blunt knife until they ping off. Make vertical slashes in the skin on both sides of the salmon from its back towards its tummy – about 2cm deep and at an angle, so you leave a flap of skin you can stuff your herbs under. Make about 6 slashes on each side of the fish. Sprinkle salt and pepper into each slash, then stuff with a pinch of your lemon-herb mix. Smooth the flaps down again and drizzle the fish all over with a light coating of olive oil. Lay it on top of your potatoes and fennel.
Take the leftover herb bunches and stuff them inside the belly cavity of the fish, then slice up one of the zested lemons and stuff these slices in there as well. Bake the fish in your screaming hot oven for 15 minutes, then turn the temperature down to 180ºC/350ºF/gas 4 and cook for another 30 minutes.
To check the fish is cooked, take a clean skewer and push it into the deepest part of the fish, just behind the head. Count to 10, then carefully take the skewer out and hold it against your top lip. If it’s nice and warm, the fish is cooked. Squeeze the juice of your remaining lemon over the top, drizzle with a little extra virgin olive oil and serve. Delicious.