Scrape the condensed milk into a 9-by-13-inch glass baking dish and sprinkle with a scant 1/2 teaspoon of fleur de sel. Cover the dish with foil and place it in a roasting pan. Add enough hot water to the pan to reach one-third of the way up the side of the baking dish. Bake, lifting the foil to stir 2 or 3 times, until the condensed milk is golden and thickened, about 2 hours; add more water to the roasting pan as necessary. The consistency of the caramel should be like dulce de leche. Don't worry if it is lumpy; it will smooth out as it chills.