Heat the oven to 400°F. In a sauce pot over medium heat, warm 2 tablespoons oil until hot but not smoking. Add the couscous, season with salt, and cook, stirring occasionally, until just toasted, 3 to 4 minutes. Add 1½ cups water and season with salt. Bring to a boil, reduce to a simmer, cover and cook until the liquid has been absorbed and the couscous is tender, 18 to 20 minutes.
While the couscous cooks, prepare the rest of the ingredients.

• Finely chop the basil leaves.
• Finely chop the dill leaves.
• Zest and juice the lemon.

In a mixing bowl, combine the buttermilk with dressing seasoning, the basil, dill, and lemon zest. Season with salt and pepper. Set aside half of the of the dressing.

• Peel the onion and cut it in half through the middle. Cut half into thin rings. Cut the remaining onion half in half again and then cut into thin slices.

In a mixing bowl, toss the salmon and the onion rings in half of the buttermilk dressing and let marinate for at least 5 minutes. Remove the fish and the onion rings from the marinade and place on a baking dish or sheet pan. Discard any remaining marinade. Roast in the hot oven until the salmon is firm and opaque, 15 to 18 minutes. Meanwhile, make the salad.

• Wash the cucumber and cut into thin slices.

In a mixing bowl, combine the remaining buttermilk dressing with the lemon juice. Toss with the cucumber and the remaining onions. Season with salt and pepper.

Transfer the salmon to individual plates and top with the cucumber-onion salad. Serve with the couscous.