Preheat the oven to 200°C.
Season the tuna steaks with sea salt and rub in olive oil.
Sear the tuna in a hot heavy-based frying pan for 30 seconds per side until browned.
Transfer the steaks to a tray and roast in the oven for 1-2 minutes until slightly pink.
Whisk the mustard and some olive oil in a bowl, then stir in finely chopped anchovies.
Pick the herb leaves and place them on a board with the capers.
Finely chop the herbs and capers and add them to the mustard mixture.
Crush the garlic with salt into a fine paste and mix into the bowl.
Whisk in the remaining olive oil until the sauce reaches a spooning consistency.
Stir in the lemon juice immediately before serving to maintain color.
Serve the tuna with lemon wedges, salsa verde, and parsley sprigs.