Heat oven to broil. Place nonstick foil on a rimmed baking sheet and set aside.
Bring 1-1/4 cups water to a boil in a medium-size pot. Add 1/2 teaspoon of the salt and the couscous. Cover and cook 8 minutes, stirring occasionally; set aside.
Meanwhile, stir together bread crumbs and 1 tablespoon dill; set aside. Sprinkle salmon with remaining 1/4 teaspoon salt and the pepper and place on prepared baking sheet. Place directly under broiler and cook for 5 minutes.
Remove salmon from oven and lower rack so its 6 inches from heating element. Brush salmon with mustard and sprinkle with bread crumb mixture, pressing to adhere. Generously spritz each fillet with nonstick cooking spray and return to oven; broil 2 minutes.
Stir remaining tablespoon dill, spinach and chicken broth into couscous; let sit 5 minutes. Serve with salmon.
