Okinawa Purple Sweet Potato Gratin


  • 6 large, purple sweet potatoes
  • 1 large white onion, finely chopped
  • 2 cloves garlic, finely minced
  • 8 tablespoons flour
  • 8 tablespoons butter
  • 4 cups whole milk
  • 1 tablespoon fresh thyme, picked and chopped
  • 1 tablespoon fresh sage, picked and chopped
  • 1 ½ cup parmigiano regianno
  • ½ tablespoon freshly ground black pepper
  • ½ teaspoon ground nutmeg
  • Salt to taste


  1. Preheat oven to 350°F
  2. Slice peeled sweet potatoes on a mandolin very thinly, place in a bowl and cover with cold water.
  3. In a medium saucepot, melt butter, add onions and garlic and cook until completely soft. Add flour and cook, stirring frequently for about 5 minutes. Add in milk and whisk continuously until completely combined. Bring to a simmer and let thicken up.
  4. Once mil mixture is thick, add in pepper, nutmeg, sage thyme and sale to taste. Cook a further 5 minutes and remove from heat.
  5. Grease a 7×9 inch casserole dish. Drain potatoes.
  6. Lay down 1/3 of the potatoes slightly overlapping. Season with salt and pepper. Spoon 1/3 of the béchamel sauce over potatoes, sprinkle with some parmigiano and repeat this two more times, finishing with the parmigiano.
  7. Cover with foil, place on a sheet tray and back in the oven for 25 minutes. Remove foil and bake for another 30 minutes. The top should be nice and golden brown. If needed, you can brown it under broil for a few minutes.
  8. Remove from over and let stand for at least 20 minutes before cutting.