Yogurt Sauce 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.
Sea Bass Preheat oven to 325°. Pour 4 Tbsp. oil in a roasting pan just large enough to fit the Sea Bass. Make a bed of herbs in bottom of pan; top with Sea Bass, skin side down. Drizzle Sea Bass with remaining 2 Tbsp. oil and season with salt. Top with tomatoes, if using. Bake until Sea Bass is just cooked through in the center (a small knife will slide easily through flesh), 25-30 minutes.
Tomatoes and Couscous Toss tomatoes with 3 Tbsp. oil, parsley, and za'atar, if using, in a medium bowl. Season to taste with salt. Set aside. 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 Tbsp. oil. Season to taste with salt. Gently fold tomatoes into couscous
Use a large spoon or fork to serve Sea Bass, leaving skin in pan. Serve with yogurt sauce and couscous.