Preheat the oven to 200°C, fan 180°C, gas 6. Put the potatoes and 1 tablespoon of the oil in a large roasting tin. Roast in the oven for 20 minutes. Add the tomatoes and cook for a further 10 minutes.
Meanwhile, in a bowl, mix together the pesto, herbs, mustard and vinegar.
Heat the remaining oil in a non-stick frying pan. Add the sardines and cook for 3 minutes on each side. Serve with the potatoes, tomatoes and salsa verde.