How to Make It
Heat the butter in a large pot over medium-high heat. Add the onions, 1¼ teaspoons salt, and ¼ teaspoon pepper and cook, covered, stirring occasionally, until tender, 12 to 15 minutes. Reduce heat to medium and cook, uncovered, stirring occasionally, until the onions are golden brown, 50 to 60 minutes.
Add the wine and cook until slightly reduced, about 2 minutes. Add the broth and 6 cups water and bring to a boil. Reduce heat and simmer for 15 minutes.
Meanwhile, heat broiler. Place the bread on a broilerproof baking sheet and broil until golden brown and crisp, 1 to 2 minutes per side. Sprinkle with the cheese and broil until melted, 1 to 2 minutes.
Divide the soup among bowls, top with the toasts, and sprinkle with the thyme.