How to Make It
In a medium bowl, cover the potato sticks with water and let stand for 15 minutes. Drain, then rinse the potatoes and pat thoroughly dry.
In a large saucepan, heat 2 inches of oil to 275°. Line a baking sheet with paper towels. Working in batches, fry the potatoes until almost tender and slightly translucent, about 5 minutes. Transfer the potatoes to the paper towels to drain. Refrigerate for 30 minutes.
Reheat the oil to 350°. In a mortar, pound the garlic with the rosemary and a pinch of salt until a paste forms. Fry the potatoes in batches until golden and crisp, about 5 minutes. Using a slotted spoon, transfer the fries to a baking sheet and immediately season with salt. Toss with the garlic-rosemary paste and serve the fries right away, with Dijon mustard alongside.