You will need
Preheat the oven to 240C/475F/Gas 9.
Put a bit of greaseproof paper on the bottom of a baking tray, rubbed with olive oil. Slash the fish fillets about half-way down and stuff the slashes with the herbs.
Slice the potatoes lengthways, just under 1 cm /.5 inch thick. Dry them off with kitchen paper and very lightly coat them in olive oil. Mix in half of your garlic, season with salt and pepper, then lay them out in one layer on the tray. Cook the potatoes in the oven for around 15 minutes until just cooked. Remove and put to one side.
Put the rest of the garlic into a pan with 2 good knobs of butter and a lug of olive oil. Fry your mixed mushrooms and season until tasty. If water comes out of them just continue cooking until it evaporates. Take the pan off the heat, squeeze in the juice from 1 lemon and stir in another knob of butter.
Now scatter the mushrooms over the potatoes and kind of rub them in on top, underneath, all over. Place your sea bream fillets on top. Now bake in the oven for 12-15 minutes, depending on the thickness of your fish.
Remove the tray from the oven, put some kitchen foil over the top and let it sit for about 5 minutes, during which time all the lovely juices will run out into the potatoes.
Serve with half a lemon, salsa verde and a glass of crisp white wine.