In a soup pot, heat butter over medium heat. Add onion, potatoes, paprika and cumin and cook, uncovered, stirring frequently, for about 8 minutes. Add 1/2 cup water and scrape the bottom of the pot to release the browned bits and juices.
Add remaining 4 cups water, Rainbow Chard, condensed soup, cilantro and 1/2 teaspoon salt. Bring to a boil, then lower the heat to lowest setting and simmer, partially covered, for 15 minutes.
For a smooth soup, purée in batches in either a food processor or blender, and return pureed soup to pot. (Or use a stick blender and blend it right in the soup pot.)
Divide rice among 4 soup bowls, and ladle soup on top of rice.
Garnish with a dollop of sour cream.