Peel the peaches and remove the pits. Cut them into chunks, transfer to a food processor and process until smooth. You can do this in batches.
Pour the peach puree in a wide and deep pot and add the sugar and one tablespoon of the lemon juice.
Bring to a light boil and cook for about 20 minutes stirring very frequently. The jam will be ready when it’s thick enough when dropped onto a cold plate.
Transfer the hot jam into pasteurized jars. Using thick gloves or a towel, secure the lids, invert the jars upside-down, and let them cool.