Combine diced peaches and minced jalapeños in a large saucepan over medium heat, then crush slightly with a potato masher until the mixture is partially broken down with visible chunks. Cook until hot and starting to release juices, about 5 minutes, stirring occasionally.

Stir in the lemon juice and powdered pectin and keep mixing until fully incorporated with no dry powder visible. Cook for 2 minutes over medium heat, with the mixture looking glossy and evenly combined.

Bring the jam to a full rolling boil over high heat, where bubbles don’t slow down when stirred. Keep boiling for 2 minutes, then add all the sugar at once and stir immediately.

Return to a full rolling boil that cannot be stirred down, then cook for exactly 1 minute while stirring constantly. The jam should thicken slightly and look smoother, with a glossy surface as it sheets off the spoon.

Ladle the hot jam into sterilized half-pint jars, leaving 1/4-inch headspace, and remove air bubbles if needed. Wipe the jar rims clean with a damp paper towel to ensure proper sealing.

Seal with lids and process in a boiling water bath for 10 minutes. After processing, let jars cool to room temperature, then store either sealed at room temperature or refrigerate if not processed.