I finally found cavolo nero, a.k.a. Tuscan black kale (dark green, really) at the local farmer's market! My quest to make my favorite ribollita soup was in my grasp. I used the recipe from Twelve, which seemed to be authentic without taking three days to prepare. Many other recipes called for making minestrone-ish soup, then adding slices of red onion and bread the next day, then reboiling it the next. Ribollita literally means "reboiled."
The finished product
You need about three pounds of kale and Swiss chard, a bit of white beans, lots of red onion, a couple of stalks of celery, a dab of tomato paste, and some salt. Getting the mound of greens home and then having to chop them down for an hour was the hardest part. A good crusty farmer's loaf of bread, well toasted, and set under a ladel of soup in each bowl made it hearty and just wonderful. Loads of antioxidants and flavor. We're on night two of it, and it's only getting better.