Prepare the fruit: Place the frozen berries in a large saucepan. Let them thaw slightly, stirring occasionally, until they become a thick slush.

Add sugar and lemon juice: Stir in the sugar and lemon juice until well combined.

Bring to a boil: Place the saucepan over medium-high heat and bring the mixture to a rolling boil.

Add pectin (optional): If using pectin, add it to the boiling mixture and stir for 1 minute.

Simmer until thickened: Reduce heat to medium-low and simmer, stirring occasionally, for 20-25 minutes or until the jam thickens. Test for thickness by dropping a small amount onto a chilled plate. If it sets quickly, the jam is ready.

Can the jam: Ladle the hot jam into sterilized canning jars, leaving about 1/4 inch headspace. Wipe the jar rims clean and secure the lids tightly.

Process in a water bath: Place the filled jars in a water bath canner, ensuring they are covered by at least an inch of water. Bring the water to a rolling boil and process for 5 minutes for half-pint jars.

Cool and store: Remove the jars from the water bath and let them cool completely on a rack. Store the jars in a cool, dark place for up to a year.

Note: For a smoother jam, you can mash the berries before adding the sugar and lemon juice. If you don't have a water bath canner, you can store the jam in the refrigerator for up to 3 weeks.