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- Preheat the oven to 200C/400F/Gas 6.
- For the tuna, season the tuna steaks with the sea salt and drizzle with the olive oil. Rub the salt and oil into the steaks.
- Heat a heavy-based frying pan until hot. Add a tuna steak to the hot pan and sear until lightly browned.
- Carefully turn over and sear the other side (this will take about 30 seconds on each side, longer if the tuna is thickly cut).
- Remove and keep to one side and repeat with the other tuna steaks.
- Place the tuna steaks in a roasting tray and place in the hot oven for another minute or two (they should still be slightly pink on the inside). Remove and keep warm.
- For the salsa verde, place the mustard and a few tablespoons of the olive oil in a bowl and whisk to emulsify. Chop the anchovies finely and add to the bowl.
- Pick the leaves from the herbs (except the basil – you can include the stems in the sauce). Pile the picked herbs onto a chopping board. Sprinkle the capers over the top.
- Chop the herbs and capers finely with a sharp knife. When finely chopped, add to the bowl with the mustard and oil mixture.
- Peel the garlic clove. Place it on the chopping board and sprinkle over the sea salt. Crush to a fine paste with a knife blade and add the paste to the bowl and mix.
- Add some of the remaining olive oil to the bowl. The sauce should have a spooning consistency, so add just enough oil – you may have some left over. Mix thoroughly.
- Just before serving, add the lemon juice to the sauce (the lemon will cause the herbs to go brown if you add it too early).
- To serve, place the tuna steaks onto plates. Squeeze over some lemon juice and serve with a wedge of lemon, a dollop of salsa verde and a sprig of parsley each.