1) Begin by putting the oil, herbs and garlic in a heavy-bottomed pot over medium heat. Bring the oil up to 185C, then remove the herbs and garlic with a slotted spoon and drain on kitchen paper. They should be nice and crackly at this stage. Reserve for garnish.
2) Using a mandoline, cut the sweet potatoes into matchsticks. Put in a large bowl of ice water to remove any starch. Soak for a few minutes and rinse with fresh cold water until it runs clear. Drain the matchsticks and pat dry in a kitchen towel, to remove as much water as possible.
3) Fry in hot oil, in small batches, until crispy and golden, 2 to 3 minutes. Transfer to paper cones, resting in cups, and season with Cajun spice mix. Peel the garlic cloves and sprinkle over the matchsticks along with the fried herbs before serving.
For the Cajun seasoning:
1) Mix all the ingredients together in a small bowl.