Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add zucchini, cover, and steam until slightly tender, 5 to 6 minutes.
Heat butter in a saucepan over medium heat; cook and stir onion in the melted butter until tender and translucent, 5 to 10 minutes.
Mix grated ginger, cumin, coriander, mustard seeds, ground cinnamon, turmeric, cardamom, cayenne pepper, sea salt, and black pepper in a small bowl. Mix spice mixture into onions until coated.
Mix zucchini into onion mixture; cook and stir until heated through, 2 to 3 minutes. Pour vegetable stock over zucchini mixture; stir well. Simmer until vegetables are tender, 8 to 10 minutes.
Pour zucchini mixture into a blender no more than half full. Cover and hold lid down; pulse a few times before leaving on to blend. Puree in batches until smooth.
Pour soup into 4 serving bowls; garnish each bowl with 1 tablespoon cilantro and 1 cinnamon stick.
