Preparation

• 1

  Preheat the oven to 325°F.
  

• 2

  Generously butter a 10-inch diameter gratin dish or cast iron pan
  

• 3

  In a large bowl, combine the hot coffee, vinegar, butter and thyme. Season with salt and pepper.
  

• 4

  Cut the sweet potatoes into thin slices and transfer them to the bowl, making sure to coat them thoroughly.
  

• 5

  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. 
  

• 6

  Pour the remaining coffee mixture into the dish and covered with well-buttered aluminum foil.
  

• 7

  Bake for 1 hour. Remove foil and continue cooking for 40 minutes at 425°F so that the top becomes crisp. 
  

• 8

  Garnish with pistachios and serve immediately.