4 large duck breasts (about 2 pounds)
Salt and pepper
4 tablespoons red miso
2 tablespoons light soy sauce or tamari
1 tablespoon sake or mirin
1 tablespoon grated orange zest
1 tablespoon grated ginger
1 garlic clove, minced
Pinch of cayenne
1 tablespoon toasted sesame oil
4 tablespoons freshly squeezed orange juice
6 ounces small green beans, topped and tailed
Lettuce leaves, for serving
1 large mango, peeled and sliced
Watercress sprigs, for garnish (optional)
3 tablespoons chopped scallions
Trim duck breasts of extraneous fat (or ask your butcher to trim them) and score the skin. Season very lightly with salt and generously with coarsely ground pepper.
Make the marinade: In a mixing bowl, whisk together miso, soy sauce, sake, orange zest, ginger, garlic, cayenne and sesame oil. Remove 1/4 cup of the marinade and combine it with 2 tablespoons orange juice to make a dressing; set aside. Add remaining 2 tablespoons orange juice to the marinade in the mixing bowl.
Lay duck in a shallow pan and pour the marinade over, making sure meat is well coated. Let sit at room temperature for at least 1 hour. If you wish, cover and refrigerate for up to 1 day; bring to room temperature before proceeding.
Prepare a bed of medium-hot coals in a grill, or heat a stovetop grill or cast-iron pan to medium hot. Cook duck breasts skin-side down for 8 to 10 minutes, until fat is rendered and skin is nicely colored. (See note.) Turn and cook on the other side for 3 or 4 minutes, until internal temperature registers 125 degrees. Remove from heat and let rest at least 10 minutes.
Meanwhile, bring a pot of well-salted water to a boil. Cook green beans for 1 to 2 minutes, until firm-tender. Drain green beans and rinse with cool water; blot dry.
Slice duck crosswise about 1/8-inch thick. Line a platter with lettuce leaves. Place several slices of duck on each leaf, along with a couple of mango slices. Arrange green beans over the top and garnish with watercress, if using. Drizzle reserved dressing over everything, sprinkle with scallions and serve.