Do not rinse or soak the potatoes, as this will wash away their starch, which is essential to the dish. A mandoline makes slicing the potatoes much easier. For the proper texture, make sure to let the casserole stand 20 minutes before serving.
1. Adjust oven rack to lower-middle position and heat oven to 425 degrees. Grease 13 by 9-inch baking dish.
2. Cook bacon in medium saucepan over medium-low heat until crisp, 10 to 13 minutes. Using slotted spoon, transfer bacon to paper towel–lined plate. Remove and discard all but 1 tablespoon fat from pot. Return pot to medium heat and add onion and 1/4 teaspoon salt; cook, stirring frequently, until onion is soft and golden brown, about 25 minutes, adjusting heat and adding water 1 tablespoon at a time if onion or bottom of pot becomes too dark. Transfer onion to large bowl; add bacon, thyme, remaining 1 teaspoon salt, and pepper. Add broths to now-empty saucepan and bring to simmer over medium-high heat, scraping bottom of pan to loosen any browned bits.
3. Slice potatoes 1/8 inch thick. Transfer to bowl with onion mixture and toss to combine. Transfer to prepared baking dish. Firmly press down on mixture to compress into even layer. Carefully pour hot broth over top of potatoes. Dot surface evenly with butter.
4. Bake, uncovered, until potatoes are tender and golden brown on edges and most of liquid has been absorbed, 45 to 55 minutes. Transfer to wire rack and let stand for 20 minutes to fully absorb broth before cutting and serving.