What makes our sweet potato tian recipe even more delightful than the Provençal original?
Preheat the oven to 325°F.
Generously butter a 10-inch diameter gratin dish or cast iron pan
In a large bowl, combine the hot coffee, vinegar, butter and thyme. Season with salt and pepper.
Cut the sweet potatoes into thin slices and transfer them to the bowl, making sure to coat them thoroughly.
Place the sweet potato slices into the gratin dish or pan in stacks of several at a time, overlapping them to form a rosette pattern.
Pour the remaining coffee mixture into the dish and covered with well-buttered aluminum foil.
Bake for 1 hour. Remove foil and continue cooking for 40 minutes at 425°F so that the top becomes crisp.
Garnish with pistachios and serve immediately.