Trim off the ends of the sweet potatoes (no need to peel them), stand them on end and slice down vertically into ½-wide planks, then into ½-inch wide fries. If the potatoes are longer than 4 inches, cut the slices in half.
Put the sweet potatoes in a wide, heavy pot (I used a 4-quart casserole pot 10 inches in diameter and 4 inches deep). Cover with the oil, place over high heat and bring to a boil. It should take about 5 minutes.
Once the oil is bubbling vigorously, set a timer for 15 minutes. Use a deep-fry thermometer to adjust the heat if needed, keeping the oil somewhere between 275 and 300 degrees.
After 15 minutes, carefully move the sweet potatoes around with a pair of long tongs to mix them around a little. Add the garlic cloves and continue frying for another 5 or 10 minutes, keeping your eye peeled that neither the potatoes or garlic gets too dark.
Test a fry —carefully— for doneness. If they are golden and crisp and tender on the inside, toss in the herbs (stand back while you do this in case of splatters) and fry for another minute or so.
Use a slotted skimmer to transfer everything to a towel-lined baking sheet. Blot briefly and sprinkle with salt. Serve right away.
A deep fry thermometer is highly recommended.Recipe inspired by Tuscan Fries in Nigellissima