Prepare the Broccoli Rabe: Bring a large pot of well-salted water to a boil. Add the broccoli rabe and cook for 5 to 6 minutes until tender. Drain, rinse under cold water, and squeeze out as much liquid as possible. Chop the broccoli rabe into ½ inch pieces.
Sauté the Broccoli Rabe: Heat the olive oil in a sauté pan over medium heat. Add the garlic and crushed red pepper flakes and cook for about 1 minute until fragrant, being careful not to burn the garlic. Add the broccoli rabe and salt, then cook for 5 to 7 minutes, stirring occasionally, until the liquid has evaporated and the greens begin to brown slightly.
Cook the Polenta: In a saucepan, bring the water and salt to a boil over medium heat. Gradually whisk in the polenta and reduce the heat to low. Cook for about 5 minutes, stirring frequently, until the polenta has absorbed the water and thickened. Remove from the heat, cover, and let stand for 2 minutes. Whisk in the olive oil until smooth and creamy.
Assemble the Dish: Spoon the polenta into serving bowls. Top with the mozzarella so it begins to melt, then add the sautéed broccoli rabe. Finish with grated pecorino Romano and freshly ground black pepper. Serve immediately.