To make the herb oil, pour the oil into a small saucepan. Add half the rosemary, thyme and sage and all the garlic. Heat gently for a few minutes, watching to make sure the garlic only slightly colours. Leave to cool, then strain into a jug or bottle.
Preheat the oven to 200C/180C Fan/Gas 6. Line a large roasting tray with two sheets of kitchen foil, leaving enough overlap so it can be folded into a loose parcel. Grease the foil with herb oil. Slash the sea bream two to three times on each side using a sharp knife. Season generously, inside and out, and stuff with the remaining herbs and half the lemon. Tuck half the remaining lemon under the fish and place the rest on top. Drizzle over more herb oil and wrap the fish in the foil.
Bake for 15–20 minutes, or until cooked through and piping hot. Unwrap and transfer to a serving platter, drizzling over any juices that have collected in the foil. Drizzle over a little more herb oil.
Meanwhile, for the garlic prawns, heat the oil in a large frying pan and add the garlic, parsley, chilli and a large pinch of salt. Cook for 2 minutes, stirring constantly, until the garlic starts to soften but not colour. Turn up the heat and add the prawns, frying briskly for 1–2 minutes on each side until they have turned completely pink. Transfer the prawns to kitchen paper using a slotted spoon.
Serve the sea bream and garlic prawns with lemon wedges and griddled asparagus.