Cook onion in 1 tablespoon oil in a 2- to 3-quart heavy saucepan over moderate heat, stirring occasionally, until golden, about 3 minutes. Add garlic and cook, stirring, for about 30 seconds. Add water and broth and bring to a boil.

Stir in couscous and raisins, then cover and remove from heat. Let couscous stand, covered, 5 minutes. Fluff with a fork and stir in parsley, lemon zest and lemon juice. Add remaining tablespoon oil and nuts, and salt and pepper to taste.