In a large saucepan, bring the vegetable broth to a boil. Stir in the couscous, cover the pan and remove from the heat. Let sit until the liquid is absorbed, about 10 minutes. Transfer to a large bowl, stir in the raisins and cover.
Meanwhile, in a large skillet, melt 2 tablespoons butter over medium heat. Add the walnuts and cook, stirring, until toasted, about 5 minutes. Transfer to a plate.
Melt the remaining 3 tablespoons butter in the skillet over medium heat. Add the sliced celery and onion and cook, stirring occasionally, until tender, about 10 minutes. Stir in the lemon peel and juice; season with salt and pepper. Stir the cooked vegetables, celery leaves and walnuts into the couscous.