Add turkey; sauté until brown on all sides, about 8 minutes.
Transfer to plate.
Add 1 cup bacon to pot; stir 1 minute.
Add next 5 ingredients. Sauté until vegetables begin to soften, about 15 minutes.
Mix in tomato paste.
Add 12 cups broth; return turkey to pot. Bring to boil. Reduce heat to medium. Simmer until turkey and vegetables are very tender, about 45 minutes.
Transfer turkey to large plate; cool.
Cut meat into bite-size pieces, discarding all skin, bones, and tendons. Working in batches, puree chowder in processor until smooth. Return puree to same pot; add diced turkey. Thin chowder with more broth by 1/4 cupfuls if too thick. Season with cayenne, salt, and pepper. DO AHEAD Can be made 2 days ahead. Chill uncovered until cold, then cover and keep chilled. Rewarm before serving.
Cook onions and brussels sprouts in pan of boiling salted water until tender, about 5 minutes.
Drain; peel onions.
Cut onions and brussels sprouts in half through core. do ahead Can be made 2 days ahead. Cover and chill.
Sauté 1 cup bacon in skillet over medium heat until crisp.
Transfer to paper towels.
Pour off all but 1 tablespoon drippings.
Add onions and brussels sprouts to skillet. Toss until heated through.
Ladle chowder into bowls. Top with onions, brussels sprouts, bacon, and fresh sage.