Line a rimmed baking sheet with aluminum foil. Peel 2 large garlic cloves; cut off a thin sliver from one clove. Rub both sides of 4 bread slices with the cut side of the garlic. Arrange the bread on the prepared baking sheet and set aside. Thinly slice both cloves. Drain and rinse 1 can chickpeas.
Heat 2 tablespoons of the olive oil in a Dutch oven or high-sided skillet over medium heat until shimmering. Add the garlic and cook, stirring, until very lightly golden-brown, 1 to 2 minutes. Add 1 teaspoon harissa (or 1 teaspoon tomato paste and 1/4 teaspoon red pepper flakes) and cook, stirring, until it begins to darken, about 1 minute.
Add the beans, 1 (14-ounce) can tomatoes and their juices (breaking the tomatoes into pieces if they’re whole), 1/2 cup water, 3/4 teaspoon kosher salt, 1/2 teaspoon Italian seasoning, and several grinds black pepper. Bring to a simmer and cook, lowering the heat if it begins to simmer too rapidly, until the liquid is reduced and a spoon leaves a clean trail when scraped against the bottom of the pot, 12 to 15 minutes. Meanwhile, position an oven rack 6 inches from the broiling element and heat the broiler to HIGH. As it preheats, thinly slice 4 ounces cheese.
In the last minute of cooking for the chickpeas, stir in a few handfuls of leafy greens, tearing larger leaves into smaller pieces as you add. Remove from the heat. Taste and season with more salt and pepper as needed.
Brush both sides of the bread with the remaining 2 tablespoons olive oil. Broil until the bread is golden-brown, 1 to 2 minutes. Remove from the oven.
To assemble, flip the slices over. Divide the chickpea mixture between the slices, then top with the cheese. Return to the oven and broil until the cheese is melted and browned in spots, 1 to 3 minutes more. Cut each slice in half and serve warm.