"I first tasted pastrami-spiced, smoked duck breast about 15 years ago, and ever since I've wondered how it would be used in a classic Reuben sandwich. I never got around to actually trying it, but this is close, and I'm going to say, way, way better."
Cut shallow slashes diagonally across the fat-side of each duck breast to create cross-hatch marks.
Whisk 1 tablespoon olive oil, 1 tablespoon black pepper, coriander, 1 teaspoon kosher salt, paprika, dry mustard, and cayenne pepper together in a large bowl until a wet-rub consistency. Rub spice mixture onto all sides of duck breasts. Set duck into bowl, cover the bowl with plastic wrap, and refrigerate, 8 hours to overnight.
Pat excess rub off of duck with paper towels. Season on all sides with kosher salt.
Whisk 1/2 cup olive oil, sherry vinegar, Dijon mustard, salt, and black pepper together in a bowl until creamy and smooth. Toss cabbage and Russian dressing together in a separate bowl.
Heat vegetable oil in a skillet over medium-high heat. Cook duck, skin-side down, in hot skillet until skin is crispy and caramelized, 5 to 6 minutes. Reduce heat to medium, flip ducks breasts, and cook until pink in the middle and meat springs back when touched, about 5 minutes more. Remove to a plate to rest. Pour rendered duck fat from the skillet through a strainer into a bowl.
Heat 1 tablespoon reserved duck fat in a nonstick skillet over medium heat. Place bread in skillet and cook until lightly toasted, about 1 minute. Flip bread slices and cook until toasted on the other side, about 1 minute more. Sprinkle cheese on each piece of bread, flip slices, and cook until cheese is melted and crisped, about 30 seconds.
Slice each duck breasts into 5 pieces. Place each piece of toast on a plate and top with sliced duck breasts. Drizzle Dijon mustard mixture over the top and serve with cabbage.