How to Make It
In a large bowl, toss the shredded cabbage with the salt and sugar. Using your hands, crush and squeeze the cabbage to release the liquid. Let the cabbage stand for 10 minutes, crushing and squeezing until very soupy. Transfer the cabbage to a colander. Rinse briefly with cold water and squeeze out as much liquid as possible; you should have about 1 cup of cabbage. Transfer the cabbage to a medium bowl and add the vinegar, red and yellow peppers, onion, scallions, poppy seeds and 1/2 tablespoon of the mustard. Season the slaw with pepper.
Preheat a broiler. In a small bowl, combine the remaining 1 tablespoon of Dijon mustard with 1 tablespoon of the olive oil. Brush the pastrami slices with the mustard-oil mixture and the bread with the remaining 1 tablespoon of olive oil. Transfer the bread to a baking sheet and toast under the broiler, turning, until golden, about 3 minutes. Top the toasted bread with the Swiss cheese and the pastrami. Broil just until the cheese is melted and the pastrami is warmed through, about 2 minutes. Transfer the open-face sandwiches to plates, garnish with the cabbage slaw and serve immediately.