This can be finished with a dusting of sugar and a couple of minutes of broiling, for a brulee top. It can also be done using apples and prunes, or just apples. —mrsp
Test Kitchen Notes
This layered sweet potato gratin comes out of the oven caramelized on the edges and glistening with butter. The potatoes in the center are soft, their layers embedded with prunes; the ones around the edges are so crisp and sweet from the port, they taste candied. Slice the potatoes thinly -- use a mandoline if you have one -- and check the potatoes after 35 minutes in the oven. By 40 minutes, ours was perfect. - A&M —The Editors
sweet potatoes, sliced very thinly using a mandoline
Melt 1/2 pound butter and clarify. (Skim off the foam & milk solids until all that remains is clear butterfat.)
Heat a cup of port and soak ten pitted prunes until slightly plumped, about 20 minutes. Drain and chop the prunes coarsely.
Preheat oven to 450.
Brush clarified butter onto your favorite eight- or nine-inch round baking dish or oven-proof frying pan.
Put a layer of potatoes, overlapping in circles, in the dish. Brush with clarified butter, and salt and pepper. Put another layer of potatoes and about half the prune pieces. Salt and pepper. Brush with clarified butter. Repeat two more layers, embedding the rest of the prunes at the end and brushing each layer with clarified butter and salt and pepper. You can do four layers of potatoes or six, it's up to you. Pack the potatoes tightly. If there's a little butter left at the end, it's no big deal.
Bake at 450 for 45-60 minutes until crisp and tender.
Remove from oven when crisp and tender. After a few minutes, flip onto a serving plate.