Place salmon in a medium skillet; cover with cold water. Add lemon zest; season with salt and pepper. Bring to a boil over high. Cover, remove skillet from heat, and let stand until salmon is opaque throughout, about 15 minutes.
Meanwhile, in a blender, combine mayonnaise, lemon juice, garlic, 1/4 cup tarragon, 1/4 cup parsley, and 2 tablespoons water. Season with salt and pepper. Blend until smooth; set dressing aside.
In a large bowl, toss lettuce with tomatoes and remaining herbs. Divide salad among four plates; top with poached salmon, and drizzle with dressing.