Heat the oil in a medium saucepan over medium heat. Add the onion and cook, stirring, until tender, 3 to 5 minutes. Add the potato, thyme, garlic, rosemary, 1/2 teaspoon salt and a few grinds of black pepper and cook, stirring, until the potatoes are well coated, 2 to 3 minutes more. Pour in the broth and bring to a simmer.
Add the zucchini and simmer until the potatoes are tender, about 15 minutes, adjusting the heat as needed to maintain a simmer. Stir in the kale and cook 5 minutes more. Remove from the heat, discard the thyme and rosemary and let cool at least 5 minutes. Carefully puree until smooth with a handheld immersion blender or in 2 to 3 batches in a blender (do not fill the blender more than halfway full per batch).
Stir in the heavy cream and reheat the soup if necessary, adjusting the consistency and seasoning with additional water, salt and pepper.
Serve hot sprinkled with cayenne or paprika and topped with sliced kale.
When blending hot liquid, first let it cool for 5 minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters and pulse until smooth.
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