With the rack in the lowest position, preheat the oven to 170°C (325°F).
In a saucepan over medium heat, soften the onion in
45 ml (3 tablespoons) butter. Add the wine and bring to a boil. Simmer gently until the wine has nearly evaporated. Add the rosemary. Season with salt and pepper. Set aside.
Butter a 28 x 18-cm (11 x 7-inch) baking dish.
Slice the potatoes finely with a mandolin. Season with salt and pepper (salt lightly if using canned broth, as it is already quite salty). Spread half in the baking dish. 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 and bake until the potatoes are tender, about 2 1/2 hours. Let rest for 15 minutes before serving.
Before ovens became common in French homes, household cooks would take prepared dishes to the local baker (boulanger in French) for slow cooking in his bread oven. These “baker’s oven potatoes” are a classic example.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.