In a small skillet, toast the cumin and coriander over moderate heat until fragrant, about 30 seconds. Transfer the spices to a small bowl to cool. Stir in the olive oil, lemon juice and garlic and season with salt and pepper.
In a large bowl, toss the watercress with the mint, walnuts and the dressing. Sprinkle the pomegranate seeds over the salad and serve.