Preheat oven to 350 degrees F. Line a baking sheet with parchment paper.
In a saucepan, add broccoli florets and just cover with water, bring to a boil. Cook covered until the water has evaporated and broccoli is mushy. Mix in 2 tablespoons butter and 1 teaspoon salt. Set aside.
Meanwhile, in another saucepan melt 1 tablespoon butter. Add in onion and cook until softened, about 2 minutes. Add in couscous and cook till toasted, about 2 minutes. Add in chicken stock and bring to a boil then cooked cover for 12 minutes, or until all the water is absorbed. Mix in mashed broccoli and stir until completely combined. Season with salt and pepper, to taste.
In a small mixing bowl, whisk together mayo, mustard, lemon juice, Worcestershire sauce, dill, and salt. Use about 1 tablespoon of special sauce to coat the salmon fillets and reserve the rest of the sauce to serve alongside.
In the preheated oven, cook salmon on baking sheet, for about 15 to 20 minutes, until cooked all the way through. Serve with couscous broccoli mash and drizzle with the extra special sauce.