Gather the ingredients for your peach jalapeno jam.
Peel and slice the peaches. Then, remove their pits. To make peeling easy, you can give the peaches a quick blanch in boiling water and dip them in an ice bath.
Combine the sliced peaches and sugar in a nonreactive pan, such as a stainless steel pan. Let them sit for 1 to 2 hours or until the peaches look noticeably softer and the sugar has dissolved.
Place a plate in the freezer. We'll use this to test the jam.
Stir the lemon juice and diced jalapenos into the peach and sugar mixture.
Bring to a boil over high heat, then reduce to low and simmer, stirring occasionally to prevent scorching on the bottom of the pan. Foam will rise to the surface during this stage, and it's up to you whether to skim it with a spoon to avoid a cloudy finished product.
After 40 minutes or when the bubbles have slowed, pull the frozen plate from the freezer and spoon a small amount of peach jam onto it. Pop it back into the freezer for 5 minutes.
The peach jalapeno jam has properly gelled if it holds its shape well when you tilt the plate. If it's still runny, keep cooking and test it every 5 minutes.
Finally, pour the jam into the sterilized jars. Timing is important because you want to transfer the jam into the hot jars while it's still hot to avoid thermal shock. You can prepare the jars when the jam is about 20 minutes away from being ready!
Leave 1/4 inch of headspace and cover the jars with their lids and rings. Set aside to cool at room temperature, then refrigerate for up to 3 weeks. Enjoy!