Cook Couscous: In a saucepan, add water, couscous, and salt. Stir and bring to a boil. Cook for about 8 minutes until couscous is tender. Drain and set aside.
Prepare Sea Bream: Cut sea bream into fillets and score the skin side to help even cooking and flavor penetration. Sprinkle olive oil and salt on both sides. Heat a non-stick pan with a tablespoon of olive oil.
Cook Sea Bream: Place sea bream fillets skin side down in the hot pan, pressing gently to keep flat. Cook on high heat for about 2.5 minutes on the skin side until crispy, then flip and cook the flesh side for about 45 seconds. Add butter to the pan away from the skin to baste the fish without softening the skin. Remove fish from pan and keep warm.
Make Lemon Sauce: In the same pan, add flour and garlic clove, stirring for about 30 seconds. Add water gradually while whisking to incorporate flour and deglaze the pan. Remove garlic once flavor is infused. Add lemon juice, white wine, and thyme sprig. Bring to a boil and simmer until sauce thickens.
Finish Sauce: Remove thyme sprig. Turn off heat and stir in mustard, salt, and pepper. Add half of the chopped parsley to the sauce and mix well.
Prepare Couscous: In a bowl, combine drained couscous with olive oil, salt, lemon juice, and the remaining chopped parsley. Mix gently to combine.
Serve: Plate the couscous and top with sea bream fillets. Pour the lemon sauce alongside or serve on the side to keep fish skin crispy. Garnish with lemon slices.