- With the rack in the lowest position, preheat the oven to 325°F (165°C). Butter an 11 x 7 inch (28 x 18 cm) baking dish.
- In a saucepan over medium heat, soften the onion in 3 tbsp (45 ml) of butter. Add the wine and bring to a boil. Simmer gently until the wine has evaporated. Season with salt and pepper. Add the rosemary. Set aside.
- Using a mandoline, slice the potatoes finely with a mandolin. Spread half of the potatoes in the baking dish. Season with salt and pepper. Spread the onion reduction mixture on top and cover with the remaining potatoes. Add the broth and dot with the remaining butter. Cover with aluminum foil.
- Bake until the potatoes are tender, about 2 hours 30 minutes. Let rest for 15 minutes before serving.