In a small saucepan, heat the sugar and water over high heat until the sugar completely dissolved. Remove from heat and let cool completely.
With a microplane, zest 1 grapefruit over an 9 x 13 x 2-inch rectangular glass or ceramic baking dish (do not use metal one to avoid reaction with the citrus). Stir in the grapefruit juice, Rosé Champagne, the cooled syrup, and fleur de sel.
Cover and carefully transfer the dish on a leveled surface in the freezer and freeze for one hour. With a large dinner fork, rake the mixture until loosen and return it to the freezer.
Continue to rake the mixture every 30 minutes or until it's firm and granular.
Spoon Frosé into stemmed glasses, garnish with fresh mint leaves, and serve immediately. Enjoy!