Okinawan Sweet Potato Gratin

Night before last I made a dinner worth remembering for some company from Colorado, a home made maple smoked ham with sauteed local crook neck squash and Okinawan (purple) sweet potato gratin.

Several requests have come in for the gratin recipe, so here it is. The Okinawan sweet potatoes are available in many parts of the country now, I even have a friend in Texas who has sourced them there. However, most are grown here in Hawaii, in fact most are grown just a few miles north of us in the deep dark soil of Onomea. As you can imagine they are not only plentiful, but also at their freshest here. In this post you can read how we sometimes volunteer to glean the fields after the perfect potatoes have been harvested for the market. We donate most of them to the Food Basket (our local food bank) and the bonus is that we can also bring home some to cook up.  

The exterior of these sweet potatoes is white and the interior is purple, the less mature ones are more pinkish purple. They can be used in a multitude of ways and are not only nutritious, but absolutely delicious. Everything from chips to desserts are made with them here. One of my favorite ways to enjoy them is simply baked with a little butter, fresh chives and black pepper. Of course it is difficult to compare anything to the Okinawan Sweet Potato Haupia (Coconut Custard) with Mac Nut Crust that I make a few times a year.

The gratin is easy to make and really delicious. Of course if you only have traditional sweet potatoes, you can do the same recipe with them.

Okinawan Sweet Potato Gratin

About 2# of Okinawan Sweet Potatoes
One large Sweet Maui Onion

2 tablespoons olive oil
3 tablespoons butter
2 tablespoons flour
4 sprigs of fresh thyme
Sea salt and freshly ground pepper
One nutmeg
1 cup freshly grated Parmigiano-Reggiano
1 1/2 cups of heavy cream

Pre-heat oven to 400 degrees

Peel and thinly slice the sweet potatoes and place in water till ready to assemble. This can be done up to 2 hours before assembly.

Thinly slice the onion and sauté in the olive oil till slightly caramelized. Add the butter and turn off the heat, the butter will melt. Add the flour and stir.

Strain the potatoes and place a layer of them in the bottom of a 2 quart casserole  (using about 1/3 of the potatoes).

Add a layer of onions (1/2 of them)  pull leaves from the thyme and sprinkle about (using 1/2 of them)

Sprinkle with a little salt and pepper. I like to be generous with the pepper.

Using a rasp or grater, grate some nutmeg over the top ( a slight bit, but so each bite will have a little.)

Top with 1/3 of the Parmigiano-Reggiano

Repeat with 1/3 of the potatoes, the remaining onions, 1/3 of the cheese, thyme, salt,  pepper & nutmeg.

Top with the remaining potatoes.

Pour the cream over the potatoes until you can just see the level of the cream below the top layer of potatoes.

Top with the final third of the cheese and some more black pepper.

Bake for 40 minutes. A crust should have formed on the top of the potatoes and the interior should be creamy and soft. Allow the dish to rest for 15 minutes before serving.