In a large pot with a lid, combine the onion, bell pepper, garlic, zucchini, and vegetable broth.

Bring to a boil over medium heat.

Reduce heat, cover, and simmer for approximately 20 minutes, or until the zucchini is very tender.

Remove from heat and purée the soup with an immersion blender until smooth (or carefully transfer to a blender in batches).

Stir in salt, onion salt, paprika, and black pepper until well combined.

Pour into bowls and top with a dollop of sour cream, grated Parmesan, croutons, and chopped parsley. Salt and pepper to taste.