Heat the oil and butter in a large saucepan over medium heat. Add the onion and cook over medium-low heat for 6 to 8 minutes, stirring occasionally, until tender but not browned. Add the stock and bring to a boil. Stir in the couscous, 1 teaspoon salt, and 1/2 teaspoon pepper and remove from the heat. Cover the pot tightly and allow the couscous to steam for 10 minutes. Fluff the couscous with a fork and stir in the mint and pine nuts. Taste for seasonings and add about 1 teaspoon salt, depending on the saltiness of the stock, and 1/2 teaspoon pepper. Serve hot.