Tip:Although a Dutch oven (a deep, straight-sided flameproof casserole with a tight-fitting lid) is traditional for this recipe, a straight-sided sauté pan works, too. If you use a sauté pan, it should be about 3 inches deep and no more than 9 inches in diameter. That way, the vegetables will be snugly packed. Be sure it has a lid. Be extra careful when you take a sauté pan out of the oven: The handle will be extremely hot, so tie a kitchen towel around it to prevent burning yourself or others who might try to touch the pan, not realizing that it's been in the oven.
Heat the oven to 375°F. Heat the olive oil in a medium (9-inch) Dutch oven over medium heat. When the oil is hot, add the pancetta. Sauté until it just begins to crisp and turn brown, about 4 min. Add the onions, thyme, bay leaf, salt, and pepper. Cook, stirring occasionally and more frequently toward the end, until the onions are deep golden, about 14 min. Meanwhile in a small saucepan, bring the chicken broth to a boil and then remove from the heat. Add the potatoes to the onions and cook, stirring for a minute or two. Pour the hot broth over the potatoes and onions and bring the pot to a boil, stirring with a wooden spoon to scrape up the browned bits from the bottom of the pan. Cover the pot, put it in the oven, and bake for about 15 min. Uncover the pot, gently and evenly push the potatoes down with a spatula, and continue to bake uncovered until the potatoes are completely tender and have started to brown, about another 25 min. Discard the bay leaf and thyme before serving.