Grill the salmon:
Heat a grill pan over medium-high heat. Coat the salmon with olive oil and season with salt and pepper. Grill about 5 minutes per side, depending on how thick your salmon filet is. Remove and let cool.
Make the couscous:
Put the couscous into a bowl and pour over 2 cups boiling water. Add 1 tablespoon olive oil, juice of 1/2 lemon, and salt and pepper to taste. Stir gently, cover, and let sit until the water is absorbed, about 15 minutes. Fluff with a fork, making sure to separate all the grains. Add the tomatoes, cucumbers, scallions, watercress, herbs, remaining olive oil, and remaining lemon juice; season with salt and pepper. Gently toss the couscous to combine all the ingredients.
Make the dressing:
Combine all the ingredients in a blender and process until smooth, green, and creamy. If necessary, thin the dressing with a tablespoon or two of warm water. Taste and adjust seasoning.
Assemble the salad:
Put a large piece of lavash bread onto a plate and pile on some of the couscous. Break the salmon apart by hand and place a big chunk on top of the couscous. Drizzle over some of the dressing and roll up the wrap.