Braised Kale and White Beans with Parmesan

¼ cup olive oil1 small fennel bulb, trimmed, cored and small-diced1 small yellow onion, small-diced2 teaspoons minced fresh rosemary or thyme5 garlic cloves, minced¼ teaspoon red-pepper flakes, plus more to taste1 large or 2 small bunches escarole, kale or Swiss chard, stems removed (10 to 12 ounces)2 (15-ounce) cans cannellini beans, rinsed2 cups low-sodium vegetable or chicken brothKosher salt and black pepper1 tablespoon lemon juice½ cup shredded mozzarella (optional)3 tablespoons grated Pecorino Romano or Parmesan, plus more for servingToasted country bread, for serving.In a 12-inch skillet or Dutch oven, heat the olive oil over medium. Add the fennel, onion and rosemary, and cook for 4 to 6 minutes, stirring occasionally, until tender. Add the garlic and red-pepper flakes and cook until fragrant, about 1 minute.Begin adding handfuls of the greens, cooking and stirring until leaves wilt.Add the white beans, broth and ¼ teaspoon black pepper, and stir to combine. Bring to a boil, then turn the heat to low and simmer, mashing some of the beans with a wooden spoon, until the liquid has reduced and thickened, 6 to 8 minutes.Off the heat, stir in the lemon juice, then the mozzarella, if using, and Pecorino Romano. Taste and season with salt and pepper. Divide among shallow bowls and top with more Pecorino Romano. Serve with toasted bread and a dish of red-pepper flakes on the side.