Put a large griddle pan onto the hob to heat up. Meanwhile, make the salsa verde. Put the parsley, basil, capers, anchovies, garlic and 2 tbsp oil in a small food processor and whizz until smooth. Season and add a squeeze of lemon juice, to taste.
Slice the polenta, season and cook on the griddle for 2 minutes on each side. Rub a little oil on the tuna, season and cook on the griddle for 2 minutes on each side. Serve with the polenta and salsa verde.