Remove casings from sausage. In a large, heavy pot over medium-high heat, cook sausage, breaking it up with a spoon, until it is lightly browned, 4 to 5 minutes. Remove sausage and its cooking juices from the pan.

Place the pot back on the stove and heat oil over medium heat until the surface begins to move. Add leeks, garlic, fennel seeds and thyme to the pan and cook for 5 to 6 minutes, or until leeks are tender. Add white wine and simmer for 1 minute. Stir in clam nectar, chicken or vegetable broth, tomatoes and sausage with their juice. Bring mixture to a simmer. Cook for 10 minutes and then taste for seasoning; adjust with salt and pepper.

Add mussels, clams and fish. Toss in greens and basil. Bring mixture back to a simmer over medium heat, cover the pot and let it cook for 5 to 7 minutes, until clams and mussels pop open. (Discard any that do not open.)

Spoon soup into heated bowls, top with a splash of vinegar and scatter with chopped parsley.

Special notes:

Most mussels have what is commonly called “the beard,” also known as byssal threads. The “beard” on a mussel is comprised of many fibers used to attach the mussel to rocks, pilings, and so on. To remove the beard, using a dry towel, grasp the beard and give a sharp yank out and toward the hinge end of the mussel.