In a large cast iron skillet, melt the tallow over medium-high heat. The temperature of the oil should be fairly high, that way it will fry and crisp up the chips instead of simply cooking them (=soggy chips) (=no bueno).
While the tallow heats up, use a mandolin to slice the sweet potatoes into super thin slices. You will notice from the photos that I have no mandolin. Just this super cool knife I've been telling you about on Facebook. By all means, the knife works to slice the sweet potatoes. But I'm telling ya, it's about time for me to splurge on one of those beautiful and useful mandolines. It's impossible for this untrained hand to get a super consistent thin slice from the potatoes, resulting in some super thin pieces and some thicker pieces. This results in slightly inconsistent results. Regardless of whether you have a mandoline or a sharp knife, I'm sure you understand the goal here. Slice those spuds.
Gently place a few of the sweet potato slices into one even layer in the hot oil. Don't overcrowd. It will take a few batches to fry them all this way, but that's okay. It'll really help to make 'em crispy when they have plenty of delicious fat to fry in.
When the potatoes begin to slowly curl up at the edges and brown (this will take 2-3 minutes), quickly flip them over. Allow the other side to gently brown as well. Don't burn them. Gently brown, I say.
When completely crispy and delicious, move them over to a towel-lined plate to dry. Quickly sprinkle each batch with a generous pinch of sea salt, freshly ground black pepper, and minced rosemary.
Repeat the steps until no more sweet potato slices remain.