1. Set the rack in the middle of the oven. (If slow-roasting fennel and tomatoes, set the second rack a few inches below.) Preheat the oven to 250°F.
2. Heat the oil in a heavy, 6-quart stove-to-oven casserole or Dutch oven. Cook the mushrooms over medium-high heat, stirring frequently, for 1 minute. Sprinkle with the salt (this will force the mushrooms to give up some liquid), and continue cooking and stirring until the mushrooms are browned and tender, 3 to 4 minutes longer.
3. Toss in the leeks, garlic, and rosemary, and cook over medium-high heat, stirring frequently, until the leeks have wilted, about 3 minutes. Add the broth and olives and bring to a boil. Add half of the escarole, cover, and cook until it wilts, about 1 minute. Stir well. Add the remaining escarole and repeat.
4. Turn off the heat. Push the vegetables aside and add the beans, taking care that they are buried under the escarole and covered with liquid. Cover, transfer the casserole to the oven, and bake until the beans are tender, 1½ to 3 hours. Toward the end of cooking, add salt if needed and pepper to taste. (The stew may seem soupy at first, but will quickly thicken as it stands.)
5. Just before serving, prepare the gremolata: Use a standard swivel vegetable peeler with very gentle pressure to remove the lemon zest (the yellow part only) in strips. (I find this works best when I move the peeler back and forth as I’m moving it down.) Place the strips on a chopping board with the parsley and garlic and finely chop them together. Cut 4 thin slices of the lemon to use as garnish.
6. To serve, stir in enough Balsamic Syrup to sharpen the flavors. Ladle the stew into large, shallow soup bowls. (Pasta bowls work well.) Float the lemon slices in the middle, and sprinkle a generous amount of gremolata on top.
Add finely chopped fennel stalks when you add the beans. Include chopped fennel fronds in the gremolata.
Substitute large lima beans for the cannellini. They will expand to the size of small potatoes.