Prepare the Nectarines: Wash the nectarines thoroughly. Cut them lengthwise around the pit, twist to separate, and remove the pits. Roughly chop the flesh.

Simmer the Fruit Base: Place the chopped nectarines, brown sugar, water, and lemon juice into a large, heavy-bottomed saucepan or Dutch oven. Bring the mixture to a gentle simmer over medium heat, stirring occasionally until the sugar is dissolved.

Cook Down: Reduce the heat to low, cover, and let the fruit cook for about 45-60 minutes, stirring every 15 minutes to prevent sticking, until the nectarines are very soft and easily mashable.

Spice and Blend: Stir in the ground cinnamon, nutmeg, cloves, and salt. Remove the mixture from the heat. Carefully transfer the warm fruit mixture into a blender or use an immersion blender to process until completely smooth. For an extra silken texture, press the puree through a fine-mesh sieve.

Thicken the Butter: Return the smooth puree to the saucepan over low heat. Cook uncovered for another 20-30 minutes, stirring frequently, until the fruit butter has visibly thickened to a spreadable consistency. It should cling to the back of a spoon.

Cool and Store: Remove the fruit butter from the heat and allow it to cool slightly. Transfer to sterilized jars. The fruit butter will thicken further as it cools. Store sealed in the refrigerator for up to two weeks or in a sterilized canning jar water bath for long-term storage.