Preheat the oven to 200C/400F/Gas 6.
For the crust mixture, place the butter, parsley, basil, lemon zest, pine nuts and breadcrumbs into a food processor and blend to a paste.
For the swordfish, lightly oil the swordfish steaks with the olive oil, then place into a very hot pan to cook for one minute on each side.
Remove the swordfish steaks from the pan and place onto a baking tray.
Season the steaks with salt and freshly ground black pepper and squeeze on some lemon juice.
Spread the crust mixture all over each swordfish steak, then transfer to the oven to roast for 10-12 minutes, or until just cooked through.
For the mashed potatoes, place the mashed potatoes, the butter and the garlic into a large bowl and mix together well.
Add the chives and season, to taste, with salt and freshly ground black pepper.
To serve, place a large spoonful of mashed potato onto each of four plates. Top each mash portion with a swordfish steak and pour over any sauce from the roasting pan.
Drizzle with good quality extra virgin olive oil.