Preheat the oven to 450 degrees . On a rimmed baking sheet, toss together the tomatoes, onion, jalapeno, garlic and olive oil and spread in an even layer. Season with salt and pepper and roast until lightly charred, 20 to 30 minutes. Toss with the olives.
Meanwhile, fill a large skillet halfway with water and bring to a boil. Add 1 teaspoon salt and place the salmon skin side down in the pan. Place a lid on top of the fillets to keep them submerged. Lower the heat and simmer until the fish is opaque, about 8 minutes. Transfer to a plate and discard the skin.
Fluff the couscous with a fork and transfer to a serving bowl. Place the salmon fillets on a serving platter and top with the vegetables.