Score each piece of sweet potato with shallow (less than 1 inch) diagonal cuts, and then cut diagonally the other direction, taking care not to pierce the skin, for a crosshatch pattern.
Massage all sides of the sweet potatoes with olive oil with your hands.
Put a small glug of oil on a baking sheet and arrange potatoes, cut side down.
Roast for about 15 minutes, flip over and brush the cut side with cider vinegar.
Gently pat on 1 teaspoon of the spice rub, using the back side of a spoon to spread mixture all the way to the edges, and bake, cut side up, for another 30 or so minutes, until rub has melted and potato is tender. Enjoy hot!
Notes
It's a pretty classic dry rub recipe - make double or triple the recipe and keep in cupboard for fajitas, ribs, salmon, roasted cauliflower, wings - whatever!