You can make the salsa verde some time in advance. You can hand chop the ingredients on a board or use a food processor, but do not turn it on for too long. Chop the parsley, mint, capers, anchovy fillets and garlic together.
Scoop them into a bowl and stir in the mustard, lemon juice, olive oil and ½ tsp of salt.
If you are going to cook the tuna on a barbecue, make sure that you light it a good 40 minutes before cooking them. Brush the tuna steaks on both sides with some oil and season well with salt and pepper. If you are using a rigid cast iron griddle, place it over a high heat and leave it to get hot, then drizzle it with a little oil.
Now cook the tuna steaks for 1 minute on either side until nicely striped from the griddle on both sides but still pink and juicy in the centre.
Put the tuna on four warmed plates and spoon some salsa verde on top.
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