Combine kale and spinach in a large soup pot over low heat. Cook just until wilted, 3 to 5 minutes. Transfer greens to a blender a little at a time. Blend until finely shredded, but not pureed, adding a small amount of water if needed.
Heat oil in a saucepan over medium heat. Add onion, serrano chile pepper, ginger, and garlic. Cook until onion is medium brown, about 5 minutes. Add coriander, cumin, garam masala, and cayenne pepper. Cook for about 1 minute more. Add kale mixture and salt and stir thoroughly. Bring to a low boil, reduce heat, and simmer 5 more minutes.
