Using a mandoline or French fry cutter, cut potatoes into 1/4" thick French fries. Soak in cold water for several hours or over night. Remove from the water and pat dry with paper towels.
Heat oil to 300 F. Fry the potatoes for 3-4 minutes until soft and only very slightly colored. Remove from the oil and transfer to paper towels. Allow excess oil to drain off the fries.
Increase the temperature of the oil to 375 F. Meanwhile, in a small saucepan over medium-low heat, slowly brown the butter; add the garlic and 2 tablespoons of the rosemary. Remove from the heat and allow the garlic and rosemary to infuse the butter.
Fry the French fries in batches until golden brown, about 1-2 minutes. Transfer to paper towels and allow excess oil to drain.
Put French fries in a large bowl and toss with the Parmesan cheese and butter. Season to taste with sea salt. Garnish with remaining rosemary and serve immediately.
Recipe courtesy of Laurent Tourondel, BLT, New York.