Put the tomatoes, half the herbs, 1 tbsp capers, the garlic, lemon zest and butter in a bowl and mash together with a spoon. Alternatively, tip into a food processor and blitz until combined. The flavoured butter will keep, chilled, for up to two days.
Layer a sheet of baking parchment large enough to loosely wrap the salmon over an equally-sized sheet of foil. Place the salmon on top, then cut the fish into portions, without cutting all the way through to the skin, so the fillet remains intact. Make the portions as big or small as you like, depending on how many you’re feeding. Spread the flavoured butter over the salmon, then top with the remaining capers, the lemon slices, olives and artichokes. Wrap the foil and parchment over the salmon and scrunch the ends to seal, creating a loose parcel.
To bake the salmon, heat the oven to 200C/180C fan/gas 6. Put the parcel on a baking tray and cook for 30 mins, then leave to stand for a few minutes before unwrapping. Alternatively, light the barbecue, wait for the flames to die down, put the parcel directly on the grill and cook for 8-15 mins, or until the salmon is cooked through. Check the salmon is cooked by pushing the flesh with a fork – it should easily flake. Serve on a platter, scattered with the remaining herbs and the buttery juices poured over.