A Parmesan rind is the secret ingredient in this hearty white-bean chili—it adds tremendous richness and depth of flavor. If you don't have one, substitute the water in the recipe for homemade or store-bought low-sodium vegetable broth.
Place poblanos directly over a gas flame, and cook, turning with tongs occasionally, until charred all over, about 8 minutes. (Or char under a broiler on a rimmed baking sheet.) Transfer to a bowl; cover with plastic wrap until cool enough to handle, about 15 minutes. Using your hands, rub off charred skin, rinsing your hands frequently under cold running water. Pat poblanos dry. Slice poblanos in half, remove seeds and stems, and cut flesh into 1-inch pieces.
Heat oil in a medium saucepan over medium-high. Add onion and a pinch of salt; cook until translucent and browned around edges, about 7 minutes. Add garlic, cumin, chili powder, and coriander; cook until fragrant, 1 minute more.
Add Parmesan rind, green chiles, poblanos, and 4 cups water. Season with 1 teaspoon salt and 1/2 teaspoon pepper; bring to a boil. Reduce heat to medium and simmer until flavors meld, 15 to 20 minutes.
In a small bowl, whisk together cornmeal and 1 cup hot cooking liquid. Return to saucepan and stir to combine. Simmer until liquid thickens slightly, about 5 minutes.
Meanwhile, in a small bowl, stir together cilantro and sour cream. Set aside.
Add beans and greens to saucepan; simmer until beans are heated through and greens are wilted, 2 to 3 minutes. Season to taste with salt and pepper. Serve immediately, with herbed sour cream, radishes, jalapenos, avocado, and a squeeze of lime.
Other dark leafy greens such as kale or Swiss chard may be used in place of spinach; remove ribs before chopping.