Soak the carne seca in cold water in the fridge for 12 to 24 hours, changing the water 3 or 4 times. Cut into chunks, cover with fresh water in a pot (or pressure cooker) and simmer until very tender, about 45 minutes (20 minutes under pressure). Drain and shred finely.
Melt the butter in a large pot over medium heat. Cook the onion for 5 minutes, then add the garlic and cook 1 minute.
Add the pumpkin, cauliflower and broth. Bring to a boil, then simmer for 20 minutes, until everything is very soft.
Blend until completely smooth, stir in the cream and season with pepper and salt, going easy on the salt since the beef is salty.
Heat the olive oil in a skillet over medium-high heat and fry the shredded carne seca, stirring, for 5 to 7 minutes until browned and crisp at the edges.
Ladle the soup into bowls and pile the crispy beef in the center. Finish with scallion greens.