Wipe out skillet. Heat remaining 1 Tbsp. oil over medium until shimmering. Add onion, fennel, plus a pinch of salt and black pepper. Cook 7 to 8 minutes or until vegetables are translucent and starting to brown. Add wine. Bring to boiling; reduce by half. Add chicken stock; return to boiling. Remove from heat. Add the lettuce, allowing hot stock mixture to wilt it.