How to Make It
In a medium saucepan, cover the barley with 1 1/2 cups of water, add a pinch of salt and bring to a boil. Cover partially and simmer over moderately low heat until the barley is tender and the water is absorbed, about 25 minutes. Stir in the chives and season with salt and pepper.
Meanwhile, in a small skillet, toast the coriander seeds over moderate heat, tossing, until fragrant, about 1 minute. Transfer to a spice grinder and let cool, then grind into a powder.
In a large saucepan, melt the butter in the grapeseed oil. Add the shallots and cook over moderate heat, stirring, until softened but not browned, about 5 minutes. Add the zucchini and ground coriander and cook over moderately low heat, stirring, until the coriander is fragrant and the zucchini just starts to soften, about 5 minutes. Add 4 cups of water and the bay leaves and bring to a boil. Simmer over moderately low heat, stirring occasionally, until the zucchini is very tender and the water is just slightly reduced, about 15 minutes. Discard the bay leaves.
Working in batches, puree the soup in a blender until smooth; add the spinach, parsley and yogurt to the last batch. Return the soup to the saucepan and season with salt and pepper.
In a medium bowl, toss the watercress with the olive oil and season lightly with salt. Ladle the soup into bowls and top with the barley and watercress. Garnish with lemon zest and serve.