Slice the nectarines but do not peel.

Toss the pieces in the lemon juice, and then stir in the sugar. The mixture will be very thick and grainy until the nectarines release their juices. Cover the mixture and refrigerate at least 4 hours, but preferably overnight.

Once the fruit has macerated in the sugar for at least 4 hours, place it in a deep stock pot or dutch oven. The mixture should only come half way up the sides of the pot to leave room for foaming as the jam cooks.

If canning, prepare a water bath canner, jars and lids.

Place the mixture on the stove, turn the heat on to high and bring the mixture to a boil, stirring continuously.

Cook on high heat, stirring constantly to prevent overflows, until the jam reaches its set point. Test for set on a plate that's been placed in the freezer, or use an instant read thermometer. (Jams "set" at 220 degrees F at sea level.  For every 500 feet above sea level, that temperature drops by 1 degree. For example, we're at about 1,000 feet in elevation, so my jams finish at 218 F.) It should take about 10 minutes to reach set, but it can vary based on the heat of your stove and the moisture in the fruit.

Once the jam reaches set, remove it from the heat and ladle it into prepared jars leaving 1/4 inch headspace.

If canning, process the jars in a water bath canner for 10 minutes (or 15 minutes above 6,000 feet in elevation). Remove jars to cool on a towel on the counter and check seals after 24 hours. Store any unsealed jars in the refrigerator for immediate use.

Properly canned nectarine jam should maintain quality on the pantry shelf for 12-18 months (refrigerate after opening).

As a refrigerator preserve, this jam will keep for 2-3 weeks. In the freezer, it'll keep 6 months, but be sure to use freezer safe straight sided jars.