Prepare the marinade by mixing all the ingredients indicated together in a bowl. Set aside until needed.
Heat a lightly greased stove-top grilling pan (or griddle) on medium heat until quite hot but not smoking.
Grill the sweet potato slices on both sides until tender but still just slightly al dente, about 8-10 minutes total. Regulate the temperature so the sweet potato grill fairly gently. You want them to have time to cook through without too much char.
If you like cross-hatched grill marks, grill the slices for about 2 minutes, turn them over and grill for another 2 minutes, then turn them over and rotate them 90 degrees, grill another 2 minutes, then turn them over again for a final 2 or 3 minutes, until done.
As the sweet potato slices are done, transfer them to a bowl, then nap the slices with some of the marinade. Repeat until all the slices are done, adding all the remaining marinade. Mix everything well.
Let the grilled sweet potatoes rest in the marinade at least 30 minutes before serving. They are better if left for several hours or even overnight.
