How to Make It
Rub the duck breasts all over with the soy sauce and 1 tablespoon of the sherry. Let marinate at room temperature for up to 1 hour.
Cook the broccoli rabe in boiling salted water until bright green, about 2 minutes. Drain in a colander. Let cool, chop coarsely, and set aside.
Season the duck breasts on both sides with salt and pepper. Heat a large nonreactive skillet over high heat. Add 1 teaspoon of the olive oil and when it is almost smoking, add the duck breasts, skin side down. Reduce the heat to moderate and cook until the skin is brown and crisp, about 4 minutes. Turn the breasts and cook the other side until browned, about 3 minutes for medium-rare. Transfer the duck to a plate and pour the fat from the skillet.
Heat the remaining 2 tablespoons olive oil in the skillet. Add the onion and bell pepper and cook over moderate heat, stirring, until softened, about 5 minutes. Stir in the garlic and anchovy paste and cook for 1 minute. Stir in the broccoli rabe, season with salt and pepper. Add the remaining 3 tablespoons sherry and simmer over moderately high heat for 2 minutes. Stir in the butter; season with salt and pepper.
Return the duck breasts, skin side up, to the skillet to reheat briefly. Transfer the broccoli rabe to plates, arrange the duck breasts alongside and serve at once.