In a large saucepan of boiling water, cook the broccoli rabe for 3 minutes; drain. Reserve the pan.
In a medium nonstick skillet, heat 1 tablespoon olive oil over medium heat. Season the tuna with salt and black pepper and add to the pan; cook, turning once, until browned, about 3 minutes. Transfer to a plate. Add the onions and 1/2 cup water to the skillet and cook over high heat until the water evaporates, about 5 minutes. Lower the heat to medium, stir in 1/4 cup olive oil and cook until golden, about 5 minutes; stir in the sugar and vinegar and season with salt and black pepper. Nestle the tuna in the onions, then cover and cook for 2 to 3 minutes.
Meanwhile, using the reserved saucepan, heat the remaining 1 tablespoon olive oil and 1 clove garlic over medium-high heat until golden-brown. Add the broccoli rabe and season with the crushed red pepper and salt. Cook, stirring, for 3 minutes. Transfer to a plate and cover to keep warm.
In the same saucepan, bring 4 cups water to a boil with the remaining 3 cloves garlic. Lower the heat to medium-low and gradually whisk in the polenta; cook, stirring constantly, until thickened, 5 to 10 minutes. Season with salt and black pepper; discard the garlic, if desired. Serve with the tuna, onions and broccoli rabe.