Rinse fish and pat dry; cut into 4 equal pieces. Rub fish all over with 1 tablespoon olive oil and set pieces, skin down and slightly apart, in a 10- by 15-inch baking pan.
Bake in a 300° regular or convection oven until fish is barely opaque but still moist-looking in center of thickest part (cut to test), 14 to 16 minutes.
Meanwhile, in a 1 1/2- to 2-quart nonstick pan over medium-high heat, stir onion and red pepper in remaining 1 tablespoon olive oil until onion is limp, 3 to 5 minutes. Add broth, couscous, and coriander seed; bring to a boil over high heat. Cover pan and remove from heat; let stand until liquid is absorbed, about 5 minutes.
In a large bowl, mix orange juice, vinegar, and ground coriander. Gently stir 1/2 cup of the juice mixture into cooked couscous. Add spinach to bowl and mix to coat.
With tongs or a slotted spoon, lift spinach from bowl and divide equally among 4 dinner plates. Mound couscous on spinach. Slide a wide spatula between fish and skin and lift fillet pieces off skin; set 1 on each mound of couscous. Drizzle remaining juice mixture in bowl over fish. Serve with orange wedges to squeeze over salmon. Add salt and pepper to taste.