Prepare the fruit: Combine the chopped nectarines, brown sugar, lemon juice, and cinnamon in a large, heavy-bottomed saucepan.

Macerate: Let the mixture stand at room temperature for at least 30 minutes, stirring occasionally, to allow the sugar to draw out the juices from the nectarines.

Cook the jam: Place the saucepan over medium-high heat. Bring the mixture to a rolling boil, stirring constantly to prevent scorching. Skim off any foam that rises to the surface.

Achieve setting point: If using pectin, dissolve it in 2 tablespoons of water and stir it into the vigorously boiling jam. Continue boiling hard until the jam reaches 220°F (104°C) on a candy thermometer, or until a small spoonful dropped onto a chilled plate wrinkles when pushed.

Finish and jar: Remove the jam from the heat. Let it cool for 5 minutes before carefully ladling the hot jam into sterilized half-pint canning jars, leaving 1/4 inch of headspace.

Process: Wipe the rims clean, apply the lids and rings, and process the jars in a boiling water bath for 10 minutes to seal. Cool completely and store.