2pint-size jars or 4 half-pint sized jars with lids and bands
To peel the peaches, bring a pot of water to a boil. Cut an X at the top and the base of the fruit and then boil the peaches for 1 minute. Remove the peaches with a slotted spoon from the water and transfer to a bowl filled with ice water to stop them from cooking. Once cool enough to handle, gently rub off the skins. Cut into into 1/4-inch thick slices, discarding the pit.
Toss the peach slices in a non-reactive saucepot with the sugar and macerate for 1-2 hours or until the sugar has dissolved and the peaches are soft and juicy. Meanwhile, sterilize the jars and lids in either a pot of boiling water or the dishwasher.
Once the peaches have macerated, it’s time to make the jam. First, place a plate in the freezer. Then, stir in the lemon juice and the diced jalapeño to the peaches. Place the saucepot on the stove and turn the heat to high. Bring the pot to a boil and then turn the heat down to low, stirring occasionally. (Don’t worry if there’s foam on top of the jam. While some advise to skim it, I leave it and find it doesn’t affect the jam.)
After 40 minutes, take the plate out of the freezer and place a spoonful of the jam on the plate. After a minute, tilt the plate and if the jam doesn’t run then it’s ready. If it does run, continue to cook it while occasionally stirring for 5 more minutes and then check again. Continue to cook and test until it doesn’t run.
Pour jam into hot, sterile jars leaving a bit of headspace. Cover with lid and rings. Allow to cool and then refrigerate. I find that it can last for a few months in the refrigerator.
Alternatively, you can place the covered jars in a canning pot or stockpot, cover the jars with water, bring to a boil and then cook on high for 10 minutes. Remove the jars with tongs and then allow to cool. If you are processing this way, make sure that your lids have never been used before, as they will only seal once. These jars will not require refrigeration until after opening.