Step 1

Mix first 5 ingredients in a medium bowl until well combined. Season with salt. DO AHEAD: Can be made 3 days ahead. Cover and chill.

Step 2

Preheat oven to 325°F. Pour 4 tablespoons oil in a roasting pan just large enough to fit the salmon. Make a bed of herbs in bottom of pan; top with salmon, skin side down. Drizzle salmon with remaining 2 tablespoons oil and season with salt. Top with tomatoes, if using. Bake until salmon is just cooked through in the center (a small knife will slide easily through flesh), 25-30 minutes.

Step 3

Toss tomatoes with 3 tablespoons oil, parsley, and za'atar, if using, in a medium bowl. Season to taste with salt. Set aside.

Step 4

Bring a medium pot of lightly salted water to a boil. Add couscous and cook until tender, about 7 minutes. Drain couscous; transfer to a large bowl. Stir in butter and remaining 1 tablespoon oil. Season to taste with salt. Gently fold tomatoes into couscous.

Step 5

Use a large spoon or fork to serve salmon, leaving skin in pan. Serve with yogurt sauce and couscous.