Make the raita: In a bowl, combine the yogurt and salt. Heat the oil in a small pan over medium heat until it shimmers. Add the mustard seeds, cumin seeds, and onion seeds, if using. When they start to pop, add the oil and seeds to the yogurt. Stir, taste, and add salt, if needed.
Make the soup: Put the cilantro, halved garlic head, and 5 cups water in a large saucepan or a stockpot. Bring to a boil, reduce to a simmer, and cook uncovered for 15 min. Strain the broth.
Melt the butter in a large, heavy saucepan over medium heat. Add the onion, 2 teaspoons salt, ginger, jalapeño, cumin seeds, coriander seeds, and cardamom, if using. Cover and cook, stirring occasionally, until the onion is translucent, about 10 min.; don't let it brown. Stir in the 1 Tbs. minced garlic and the zucchini, cover, and cook, stirring occasionally, until the squash is softened, about 5 min. Add 4 cups of the broth, bring to a simmer, and cook until the zucchini is tender, about another 3 min.
Purée the mixture in a blender and strain it (see How to purée a vegetable soup).
Add the lime juice; taste and add salt if needed. Serve warm, garnished with 1 to 2 Tbs. raita and the cilantro leaves.