In a small bowl, combine the flax seed with 2 tablespoons of water and let soak for 20 minutes.
Meanwhile, in a small bowl, whisk together 3 tablespoons red wine vinegar and 1 teaspoon cumin. While whisking, drizzle in 2 tablespoons olive oil to make a smooth dressing and season with salt and pepper, to taste. Mix the flaxseed with the dressing. Set the flaxseed dressing aside.
Preheat an outdoor grill or indoor grill pan. In a large bowl, lightly coat the peppers and tomato with 1 tablespoon of olive oil and season with salt and pepper, to taste. Grill the peppers and tomato until slightly charred and soft. Let cool.
Meanwhile, in another large bowl, whisk together the remaining 3 tablespoons red wine vinegar, 1 teaspoon of cumin, and 1 tablespoon black pepper. While whisking, drizzle in 2 tablespoons olive oil to make a smooth dressing and season with salt, to taste.
Peel and seed the peppers and tomato and leaving some of the charred skin, and slice. Toss the them with the dressing.
Heat the remaining 2 tablespoons olive oil in large skillet over medium-high heat. Add the prawns and cook, stirring, until just cooked through.
Arrange lettuce leaves along outer edge of a large platter. Arrange the peppers and tomato on the lettuce. Place the watercress in the middle and drizzle flax seed dressing over the top. Add the prawns with their juice and scatter around the preserved lemon and olives. Serve.