Prepare a boiling water bath canner and 8 half pint jars.
Cut the nectarines into thin slices and place in a large, non-reactive pot.
Wash the lemons with warm, soapy water. Cut them in half and cut the halves into paper-thin half moons (poking out seeds as you slice). Add the lemon slices to the pot.
Add the sugar to the nectarines and lemons and stir to combine. Let it sit for 15 or 20 minutes, until the sugar has dissolved and the fruit is juicy.
Stir in the raisins and the lemon juice and set the pot on the stove over high heat. Bring to a boil and then reduce the heat to medium-high.
Cook, stirring regularly, until the conserve has thickened and reduced by about a third.
Near the end of cooking, stir in the walnuts.
When you're happy with the consistency of the conserve, remove the pot from the heat.
Funnel the conserve into the prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 15 minutes.
When the time is up, remove the jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortably handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.