In a medium saucepan over medium, combine the couscous and oil. Cook, stirring, until golden brown, about 4 minutes. Add 3 cups water, the lemon zest, 2 teaspoons harissa, 1½ teaspoons salt and ¼ teaspoon pepper. Bring to a simmer and cook, stirring occasionally, until the couscous has absorbed the liquid and is tender, 10 to 13 minutes.
Meanwhile, in a large bowl, toss the tomatoes with ¼ teaspoon salt and the remaining 1 teaspoon harissa.
When the couscous is tender, add it to the tomatoes along with the olives and lemon juice; toss. Taste and add additional harissa if desired. Let stand 15 minutes, stirring once or twice, then stir in the mint. Taste and season with salt and pepper.
