Step 1: Cook the Barley
In a medium saucepan, combine the barley with 1 1/2 cups of water. Add a pinch of salt and bring to a boil. Once boiling, partially cover the pan and let simmer over moderately low heat until tender and water absorbed, about 25 minutes. Stir in chives and season with salt and pepper.
Step 2: Prepare the Coriander Seasoning
Toast coriander seeds in a skillet over medium heat, tossing frequently until fragrant, about 1 minute. Cool and grind into a fine powder.
Step 3: Cook the Soup Base
Melt butter and grapeseed oil in a large saucepan. Add shallots and cook until softened, about 5 minutes. Stir in zucchini and ground coriander and cook for another 5 minutes. Add 4 cups water and bay leaves; bring to boil. Reduce heat and simmer until zucchini is very tender and liquid reduced, about 15 minutes. Remove bay leaves.
Step 4: Puree and Season the Soup
Puree soup in batches until smooth. During final batch, add spinach, parsley, and yogurt, blending thoroughly. Return soup to saucepan; season with salt and pepper.
Step 5: Prepare Toppings and Serve
Toss watercress with olive oil and season lightly with salt. Ladle soup into bowls; top with barley and dressed watercress. Sprinkle lemon zest and serve hot.