Stir together the couscous, lemon juice and olive oil. Set aside to plump and soften, 1 to 2 hours, at room temperature, stirring occasionally. It's ready when the grains are al dente.
Stir through the herbs, tomatoes, pepper and onion. Season well with salt and pepper, adding more lemon juice to taste if needed. Cover and refrigerate overnight if not using right away.
Recipe courtesy of Laura Calder