Cut a small ‘X’ at the base of each nectarine with a paring knife. Boil nectarines for 90 seconds, then place them in an ice bath until cool to the touch. Using a paring knife, peel off the outer nectarine skin. Roughly chop the nectarine flesh to a medium (½ inch or 1cm) dice.
Heat chopped nectarines, granulated sugar, water, and lemon juice over medium high heat in a medium sized saucepan until boiling.
Reduce the heat to medium. Cook, stirring often, until the fruit has thickened. To test that the jam is thick enough, place a plate in the refrigerator until chilled. Place a small dollop of jam on the chilled plate. If the jam stiffens, it’s fully cooked. If liquid runs from the jam, it needs to continue cooking.
Transfer jam to a small bowl and refrigerate for 1 hour until fully cooled.
Store in an airtight container in the refrigerator for up to 1 - 2 weeks.