Preheat the oven to 350˚F. Place the romanesco florets in a roasting tray, drizzle with 2 tbsps of olive oil, and roast for 30 minutes.
Place the prosciutto in a separate roasting pan and roast for 10-15 minutes, or until crispy.
Heat the remaining olive oil in a large saucepan. Add the chopped onions and saute at medium heat until soft and golden, about 10 minutes.
Add the romanesco and chicken stock and cook until tender about 15-20 minutes.
Place the romanesco mixture in a food processor and process until smooth.
Return the romanesco puree to the saucepan, stir in the cream, and simmer for another 5 minutes. If the soup is too thick, add one tablespoon of water to thin it to your liking.
Add sugar and season with salt and pepper to taste.
When ready to serve, ladle the soup into bowls, add prosciutto and pepper. Garnish with a few romanesco florets.