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.

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 minutes. Strain the broth.

Melt the butter in a large, heavy saucepan over medium heat. Add the onion, 2 teaspoons salt, ginger, jalapeno, cumin seeds, coriander seeds, and cardamom, if using. Cover and cook, stirring occasionally, until the onion is translucent, about 10 minutes; don't let it brown. Stir in the 1 tablespoon minced garlic and the zucchini, cover, and cook, stirring occasionally, until the squash is softened, about 5 minutes. Add 4 cups of the broth, bring to a simmer, and cook until the zucchini is tender, about another 3 min.

Puree the mixture in a blender and strain it.

Add the lime juice; taste and add salt if needed. Serve warm, garnished with 1 to 2 tablespoons raita and the cilantro leaves.