Pommes de Terre Boulangère
2 pounds (about 7 medium) firm, waxy potatoes
6 to 8 cups beef or chicken broth, or as needed
Salt and freshly ground black pepper
1 large red or white onion, thinly sliced
3 tablespoons olive oil, or as needed
6 sprigs fresh thyme
1 tablespoon sherry vinegar
1. Place potatoes in a saucepan and add broth to cover by about 1 inch. Season with 1/2 teaspoon salt and a pinch of pepper, or to taste. Bring to a boil and simmer until just tender but not falling apart, about 20 minutes. Remove potatoes from broth (reserve broth for another use) and allow to cool to room temperature.
2. Meanwhile, in a medium skillet, combine onion with 1 tablespoon fat. Place over medium-low heat and sauté until translucent, about 5 minutes. Season with salt and pepper to taste, and add 4 thyme sprigs. Reduce heat to low, and continue to cook, stirring occasionally, until onion is soft and caramelized, about 10 more minutes. Add vinegar, cook 1 minute more. Remove from heat, and discard thyme sprigs.
3. Slice cooled potatoes into rounds 1/3 inch thick. Place a large cast-iron skillet over high heat, and add 2 tablespoons fat (or as needed to provide a thick coating on bottom of pan). When fat is extremely hot, add potatoes and allow to sit without stirring or shaking until seared and crispy.
4. Turn potatoes and sear and crisp other sides. When well-browned, add caramelized onions, salt and pepper to taste, and stir to mix. Chop remaining 2 sprigs thyme (or if stems are woody, use leaves only), sprinkle over potatoes and serve.
