Preheat oven to 180°C. Toss sweet potatoes in a little extra virgin olive oil and arrange on a lightly greased baking pan. Cook for about 10 minutes, turning once, until lightly browned and tender.
Bring water to a boil and add half a teaspoon of salt. Stir in couscous with half a teaspoon of oil. Cover and take pan away from the heat. Let it stand three to four minutes or until couscous has completely absorbed the liquid and is tender. Add one tablespoon of butter and stir through with a fork to separate the grains. Stir in onion, cucumber, olives, tuna, capsicum, lemon juice and remaining oil. Toss well.
Add the sweet potatoes and season with salt, pepper and ground black pepper.