Blanch the Swiss chard or spinach in boiling water until just wilted. Drain, squeeze dry and chop roughly. Set aside until needed.

Finely chop the onion, celery, parsley and garlic together into a battuto. Sauté gently in olive oil in a braising pot, preferably of terracotta or enameled cast iron, until the vegetables are soft and the onion translucent, stirring from time to time and taking care to avoid browning. (A few drops of water and a pinch of salt will help.)

While the aromatics are cooking, cut the bodies of the squid into strips or rings. And cut up the tentacles if they're large. Add the squid to the soffritto and let it sauté with the aromatics for a few minutes to allow them to become infused with their flavor, to insaporire, as the Italians call it. Add the tomato paste (and the red pepper flakes if using) and let simmer with the liquid for a few moments to blend.

Now add the chopped greens and mix with them up with the squid pieces, adding water if they absorb all the liquid. Lower the heat, cover and let things simmer very gently for a good 20 minutes, adding water if needed to keep things a bit brothy. Test the squid to see if it's tender, if not, give it a few more minutes. Taste and adjust for seasoning

Serve hot, with toasted slices of good, crusty bread on the side.