How to Make It
In an enameled medium cast-iron casserole, toss the nectarines with the sugar, lemon juice, bay leaves and salt. Cover and bring the nectarines to a simmer over moderate heat. Remove from the heat.
Uncover the casserole. Carefully press a round of parchment paper directly on top of the hot nectarine mixture and let cool to room temperature, about 2 hours. Refrigerate overnight.
Place a small plate in the freezer. Discard the parchment paper round. Bring the jam to a simmer over moderate heat, skimming off any foam with a ladle. Cook, stirring occasionally, until the nectarines begin to break down and the juices fall off the side of a spoon in thick, heavy drops, about 1 hour and 40 minutes. Spoon 1 tablespoon of the jam onto the chilled plate and refrigerate until it comes to room temperature, about 3 minutes; the jam is ready when it thickens like jelly and a spoon leaves a trail when it’s pulled through. If necessary, continue simmering and testing.
Spoon the jam into three 1-pint canning jars, leaving 1/4 inch of space at the top of each. Seal the jars and let the jam cool to room temperature.