Over high flame, heat a heavy-bottomed pan large enough to make a pot of soup. Film it with olive oil and cook the vegetables, garlic, bay leaves, and fennel seed until the onions start to go translucent, about five minutes. Add the white beans and chicken stock and simmer for 20-30 minutes, until the vegetables are soft.
Remove the bay leaves and smooth the soup with an immersion blender (or blend it in a blender and return it to the pot). Thin the soup as necessary with more stock or water and season to taste.
For the garnish, film a sauté pan with a little olive oil and cook the linguiça. Remove the linguiça from the pan and cook the potato in the linguiça oil, browning nicely. Pour off some of the colored oil for garnish, then add the linguica, potato, and collards to the soup and simmer until the collards are bright green and just done. Portion into soup bowls and drizzle with the reserved linguiça oil. Serves four as an appetizer or two as a main course with crusty bread.