Bring the water for the stock to a boil in a large pan and stir through the concentrated stock. Keep on a low simmer.
Reserve a small handful of the watercress for garnish, then blanch the rest of the watercress in boiling lightly salted water for a couple of minutes. Drain and then run under cold water to stop the cooking. Blitz with a splash of the stock until you get a gloriously green paste. Leave to one side.
Heat the garlic-infused oil and half of the butter in a large, heavy-bottomed pan. Cook the celery for 5 minutes, until softened but not browned.
Add the rice to the pan, stirring to make sure that it’s well coated. After a minute or so it will start to crackle and look translucent at the edges – at this point add in the wine and keep stirring, cooking until the alcohol has been evaporated and the liquid absorbed.
Start adding the hot stock, a ladleful at a time, stirring continuously. Wait for most of the stock to be absorbed before adding the next ladleful – it should take around 20 mins for all the stock to be added. Taste to see if the rice is tender, but still has some bite.
Stir through the watercress puree, the remaining butter and the parmesan cheese. Put a lid on the pan and allow it to sit for a few minutes before stirring again. Season well and add in a big squeeze of lemon. Divide the risotto between bowls, top with a handful of crumbled feta, a few fresh watercress leaves and another grind of black pepper.