white onions, halved lengthwise and cut into 1/4-inch-thick slices crosswise
freshly ground pepper
dry white wine
low-sodium chicken stock
(1-inch-thick) slices whole wheat baguette, toasted and cooled
Gruyere, shaved into paper-thin slices with a cheese plane or vegetable peeler
1. Preheat the oven to 400F.
2. Heat the oil in a large saucepan over medium heat. Add the onions, thyme, bay leaves, salt, and pepper, and cook, stirring occasionally, until onions are golden (not brown) and nearly melting, about 45 minutes. Stir in wine and simmer, stirring occasionally, until reduced by half. Stir in stock and simmer, stirring occasionally, for 15 minutes.
3. Place 4 soup crocks or ovenproof bowls (no larger than 4 inches round) on a rimmed baking sheet. Place a baguette slice in each crock. Divide the soup among the bowls, and top each with a single layer of cheese slices.
4. Transfer to the oven and bake until cheese melts and is golden brown in spots and soup is bubbling, about 10 minutes. Serve immediately.