Step 1

Place the berries in a large stainless steel or enamel saucepan. Bring to a full boil over high heat, mashing the berries with a potato masher as they cook. Add the lemon juice and boil hard for 1 minute, stirring and mashing constantly.

Step 2

Add the sugar and return the mixture to a boil. Cook, stirring constantly, until it begins to look syrupy and thickens slightly, about 5 minutes. Remove from the heat and test for doneness: a candy thermometer should register 220 degrees F and the mixture should coat the back of a spoon. Run a finger--or a clean object about the width of a finger--through the coat of jam. If the jam does not run and fill the gap, it is done. Otherwise, return the pan to the heat for another 1 to 2 minutes and test again. Be careful not to let the mixture get too thick--it will thicken as it cools.

Step 3

When the jam is done, transfer it to a heatproof jar and cool to room temperature, about 2 hours. Cover with a tight-fitting lid and refrigerate. The jam will keep for up to 2 weeks.