- 12 tablespoons (1 1/2 sticks) butter, divided
- 24 cups thinly sliced onions (about 5 3/4 pounds)
- 8 cups (or more) low-salt chicken broth
- 1 1/2 cups 1/2-inch pieces torn crustless country-style bread
- 1 1/2 tablespoons (or more) Sherry wine vinegar
- 2 teaspoons finely grated lemon peel
Melt 6 tablespoons butter in heavy extra-large pot over medium-high heat. Add onions; sprinkle with salt and pepper and sauté until onions begin to soften, 15 to 18 minutes. Reduce heat to medium and sauté until onions are very tender and deep golden brown, stirring often and adjusting heat as needed, 25 to 30 minutes longer. Transfer 1 1/2 cups caramelized onions to small bowl and reserve for garnish.
Add 8 cups broth to remaining onions in pot and bring to boil. Reduce heat and simmer 25 minutes to blend flavors. Cool slightly. Working in batches, puree soup in blender until smooth. Return to same pot and add more broth by 1/4 cupfuls to thin soup to desired consistency. Season soup to taste with salt and pepper. DO AHEAD Can be made 1 day ahead. Cover and chill reserved caramelized onions. Cool soup slightly. Chill soup uncovered until cold, then cover and keep chilled. Re-warm caramelized onions and soup separately before continuing.
Melt 2 tablespoons butter in medium nonstick skillet over medium heat. Add torn bread pieces and sauté until bread pieces are crisp and golden, 5 to 6 minutes. Remove from heat and reserve.
Cook remaining 4 tablespoons butter in small saucepan over medium heat until golden brown, stirring occasionally, 3 to 4 minutes. Add sage leaves and cook until slightly crisp, 30 seconds to 1 minute. Transfer leaves to small plate; reserve brown butter in saucepan.
Stir 1 1/2 tablespoons vinegar into reserved caramelized onions, adding more vinegar by 1/2 teaspoonfuls, if desired.
Divide warm soup among 8 bowls. Spoon vinegar-seasoned caramelized onions atop soup in each bowl. Sprinkle each serving with croutons and fried sage leaves. Drizzle sage brown butter over each; sprinkle lightly with grated lemon peel and freshly ground black pepper and serve.