Prepare the raspberries: Rinse the raspberries under cold water and gently pat them dry with a paper towel. Remove any stems or debris.

Mash the raspberries: Place the raspberries in a large bowl and gently mash them with a fork or potato masher until they reach your desired consistency (chunky or smooth).

Combine ingredients: Transfer the mashed raspberries to a large pot. Add the sugar, lemon juice, and lemon zest (if using). Stir well to combine.

Cook the jam: Heat the mixture over medium heat, stirring frequently to prevent burning. Bring it to a boil, then reduce the heat to a simmer.

Test for doneness: After about 15-20 minutes, test the jam by placing a small amount on a chilled plate. If it thickens as it cools, it's ready. If not, continue cooking for a few more minutes.

Sterilize jars: While the jam cooks, sterilize your jars and lids by boiling them in water for 10 minutes. Keep them hot until ready to use.

Fill the jars: Carefully ladle the hot jam into the sterilized jars, leaving about 1/4 inch of headspace. Wipe the rims clean and seal with the lids.

Process the jars: If canning, process the jars in a boiling water bath for 10 minutes to ensure a proper seal. Let them cool completely before storing.