Heat oven to 180C/fan 160C/gas 4. Place the hazelnuts in a baking tray, then toast in the oven for 5-10 mins until golden (you can roast the garlic for the mash at the same time). Leave the nuts to cool a little. Put the cooled nuts, chilli, 50g of the watercress, 1½ tbsp olive oil, 1 tbsp balsamic and some seasoning in a food processor, then blitz until finely chopped. The mixture shouldn’t be too smooth – there should still be some texture to the nuts. Set aside.
Heat a griddle pan until smoking hot. Rub the steaks
with a little oil, then season both sides with salt. Cook the steaks on the griddle for 2 mins on each side – this will give you medium rare (cook for 3 mins for medium or 4 mins for well done, if you prefer). Remove from the griddle, then sit on a plate for 5 mins to rest. Stir any juices from the resting steak into the pesto – they’re delicious and shouldn’t be wasted.
To serve, divide the Roasted garlic mash (recipe, below) between 2 plates, top with a little watercress, then sit a steak across it. Top the steaks with a large tbsp of the pesto and drizzle the rest of the balsamic over.