1 large head garlic
3 tablespoons extra-virgin olive oil, divided
3 cups water plus 2 tablespoons, divided
8 slices stale bread, crusts removed
4 cups halved and thinly sliced leeks (about 2 large leeks) white and light green parts only
1/2 teaspoon kosher or sea salt, divided
1 cup shredded raclette or Gruyère cheese
1 tablespoon chopped fresh thyme
1/4 teaspoon freshly ground pepper
Preheat oven to 375°F.
Remove the outer skin from the garlic and place the head in a small baking dish. Drizzle with 1/2 teaspoon oil. Add 1/4 inch water to the dish. Roast, uncovered, until tender but still firm, about 30 minutes. Let cool slightly, then peel the cloves. Thinly slice and set aside.
Meanwhile, bring 3 cups water to a boil. Place bread in a shallow heatproof dish large enough to hold it in a single layer. Pour in enough water to nearly submerge it. Let stand until the bread is saturated, 3 to 5 minutes. Transfer the bread to a colander in batches and gently press out the liquid. The bread should be somewhat moist, but not dripping. Tear it into irregular pieces. Transfer to a large bowl.
Heat 1 tablespoon oil in a large skillet or Dutch oven over medium heat until shimmering. Add leeks, 2 tablespoons water and 1/4 teaspoon salt, partially cover and cook, stirring occasionally, until the leeks are tender but still bright green, 5 to 7 minutes.
Add the leeks to the bowl of bread along with the sliced garlic, 1 1/2 tablespoons oil, cheese, thyme, pepper and the remaining 1/4 teaspoon salt; gently combine with your hands or a spoon. Spread the mixture into a shallow baking dish large enough to hold it in a 1-inch layer, such as a 9-by-13-inch pan. Drizzle with the remaining 1 teaspoon oil.
Bake until crisp and golden in spots, about 30 minutes. Serve warm