1 pound collard greens, stems and tough ribs removed
5 garlic cloves, divided, 1 crushed and 4 chopped
1/4 cup extra-virgin olive oil, divided
1 medium onion, chopped
1 small red bell pepper, cut into 1/2-inch pieces
3 cups cooked white beans, 1/2 cup cooking liquid reserved
1/2 cup diced or chopped cooked ham-hock meat, sausage, chorizo, or bacon
1/4 cup freshly grated Parmigiano-Reggiano, divided
1 tablespoon finely chopped fresh rosemary leaves
Kosher salt and ground black pepper
1/4 cup medium-coarse fresh bread crumbs
In a large pot, cover the collards with salted water, then heat and boil until tender, about 30 minutes. Drain well and chop into small pieces. Set aside. Preheat the oven to 475F. Vigorously rub the inside of a 10- or 12-inch gratin dish with the crushed garlic. Discard the crushed garlic and set the dish aside.
Heat 2 tablespoons of the oil in a large saute pan over medium-high heat. Add the onion and bell pepper and cook, stirring often, until tender, about 10 minutes. Add the chopped garlic and cook, stirring, for 1 minute. Add the collard greens, stir to coat, and cook for 1 minute. Transfer into a large bowl.
Stir in the beans, meat, 2 tablespoons of the Parmigiano-Reggiano, 1 tablespoon of the oil, and rosemary. Season with salt and pepper. Add enough of the reserved bean cooking liquid to moisten, then spread the mixture in the prepared dish. Top with the bread crumbs and the remaining Parmigiano-Reggiano. Drizzle with the remaining oil. Cover the dish with aluminum foil and bake until the filling is hot and bubbly, 30 to 35 minutes. Remove the foil and bake until the top of the gratin is golden and crusty, about another 10 minutes. Serve hot.