Preheat the broiler on high. Arrange the marinated chicken thighs on a foil-lined broiler pan.
While the broiler preheats, bring a small pan of water to a boil. Dump the couscous in a large bowl and add enough boiling water to cover it by about 1/4 inch (6 mm). Cover the bowl with plastic wrap and let sit until the water is absorbed, about 10 minutes.
In another small bowl, toss the red onion with 1 tablespoon lemon juice and a small pinch of salt. Let sit for a few minutes to mellow the sharpness of the raw onion. Then add the fennel and 1/2 tablespoon oil and toss.
Gently fluff the couscous with a fork. Add the onion and fennel mixture, chickpeas, preserved lemon, olives, chopped mint, chopped cilantro or parsley, 2 tablespoons lemon juice, and the remaining 2 tablespoons oil and toss well. Taste, adding more lemon juice or salt, if desired.
Broil the chicken thighs until cooked through and golden brown, 7 to 8 minutes per side. When cool enough to handle, slice the chicken thighs into thin strips. Sprinkle with 1 tablespoon lemon juice and a pinch of salt.
To serve, if desired, turn the salad onto a large platter. Arrange the chicken on top and spoon over any juices from the broiler pan. Garnish with whole cilantro or parsley leaves and fennel fronds, if using. Serve immediately.