Preheat oven to 375˚F.
Lightly rinse fish and pat dry.
Score branzino fillet on both sides (3 on each side) and fill cavity with fennel, shallot, tarragon, sage, lemon, salt, and pepper. Season the outside of the fish with salt and pepper and roast for 18 to 22 minutes or until just cooked through and flaky.
While the branzino is in the oven, cook the couscous. Place a medium pot over medium-high heat and add oil and butter. Add shallot and garlic and sauté for 2 to 3 minutes. Add couscous and continue to sauté for an additional 3 to 4 minutes or until couscous becomes lightly toasted and has a nutty fragrance. Season with salt and pepper.
Stir in chicken stock Bring mixture to a boil before lowering heat to medium-low. Cover and simmer until all the liquid has been absorbed and couscous is light and fluffy.
Uncover and fluff with a fork. Stir in thyme, chive and lemon zest.
Pour couscous onto a platter and carefully transfer whole branzino over the bed of couscous. Top with more fresh lemon zest and serve.