Pat salmon dry with a paper towel, and season flesh evenly with ½ tsp. salt and ¼ tsp. pepper.Heat 2 Tbsp. oil in a large skillet (preferably cast-iron) over medium-high. Once hot, arrange salmon fillets, skin side-down, in the pan, and cook for 3 to 4 minutes, until the skin easily releases from the pan. Gently flip fillets, lower the heat to medium, and cook until the salmon is cooked through and easily flakes with the pressure of a fork, about 3 to 4 more minutes. Transfer salmon to a plate and cover with foil to keep warm.

Add fennel wedges to the skillet, and cook until brown on the bottom, about 3 minutes. Add tomatoes, and toss with fennel and oil. Continue cooking until fennel softens and tomatoes are blistered, about 3 more minutes.Stir in harissa and couscous; cook, stirring frequently, until the grains are lightly toasted, about 2 minutes. Add 1 ½ cups water, remaining ½ tsp. salt and ¼ tsp. black pepper; bring mixture to a boil. Once boiling, reduce heat to medium-low, cover, and cook until most of the water is absorbed and couscous is tender, about 12 to 14 minutes.

While the couscous cooks, prepare salsa by combining olives, pistachios, parsley, garlic, chili flakes, olive oil, lemon juice, and salt; mix well.

Once the couscous is cooked, remove lid and nestle salmon fillets back into pan. Continue cooking, uncovered, 2 more minutes to warm salmon. Spoon olive-pistachio salsa over salmon, and serve immediately.