- 2 tablespoons extra-virgin olive oil
- 3 medium onions, halved lengthwise, each half cut crosswise into 1/4-inch-thick slices
- 2 tablespoons fresh thyme, divided
- 1/2 pound thick-cut bacon, sliced crosswise into 1/4-inch strips
- 1 1/2 pounds Yukon Gold potatoes
- 1 teaspoon coarse kosher salt, divided
Preheat oven to 400°F. Cut off top 1/2 inch from head of garlic. Wrap garlic in foil and roast until soft, about 45 minutes. Cool. Squeeze garlic cloves into small bowl; mash with fork. Cover and reserve.
Heat oil in heavy large skillet over medium-high heat. Add half of onions and 1 tablespoon thyme and cook until slightly softened, stirring frequently, about 6 minutes. Add remaining onions and sauté until very soft, about 15 minutes. Reduce heat to medium-low and cook until deep golden brown, stirring frequently, about 40 minutes longer. Set aside.
Sauté bacon in heavy medium saucepan over medium heat until bacon is beginning to crisp around edges, about 4 minutes. Pour off drippings from pan. Add cream, 1 tablespoon roasted garlic, and 2 teaspoons thyme; simmer until slightly thickened, about 4 minutes. Cool.
Preheat oven to 400°F. Peel and thinly slice potatoes with mandoline or knife. Spoon 1/2 cup cream mixture over bottom of 8x8x2-inch metal or ceramic baking dish. Arrange 1/3 of potato slices over cream, overlapping as necessary. Sprinkle with pepper and 1/3 teaspoon coarse salt. Spread half of caramelized onions over potatoes. Spoon half of remaining cream mixture over onions. Cover with half of remaining potatoes. Sprinkle with pepper and 1/3 teaspoon salt. Spread remaining onions over potatoes. Spoon all but 2 tablespoons cream mixture over onions. Cover with remaining potato slices. Sprinkle with black pepper and remaining 1/3 teaspoon salt. Drizzle with remaining cream mixture. Sprinkle with remaining 1 teaspoon thyme. Cover tightly with plastic wrap, then foil.
Bake until potatoes are tender when pierced with knife (pierce knife through foil), about 45 minutes. Carefully remove foil and plastic wrap (steam will be released). DO AHEAD Can be prepared 2 hours ahead. Let stand at room temperature.
Increase oven temperature to 500°F. Bake until top is golden brown, about 10 minutes longer (or about 15 minutes if at room temperature). Let stand 10 minutes. Serve.