Prepare the fruit: Rinse all of the berries under cool water and gently pat them dry with a clean kitchen towel or paper towel. Remove any stems or leaves, and pick through the berries to discard any that are overly soft or bruised. Add all the prepared berries to a medium-sized saucepan.
Add sugar and citrus: Pour the granulated sugar over the berries in the saucepan. Add the lemon juice and lemon zest, if using. The lemon not only brightens the flavor but also helps the jam set more naturally thanks to its natural pectin. Add a small pinch of salt to balance the sweetness and enhance the overall flavor.
Mash the fruit: Using a potato masher or the back of a large spoon, gently mash the berries right in the pot. You don’t need to break them down completely—just enough to release some of their natural juices and help the mixture start to come together.
Heat and stir: Set the saucepan over medium heat. As the mixture heats up, begin stirring frequently with a wooden spoon or heatproof spatula to prevent the sugar from settling and burning at the bottom. Within a few minutes, the berries will begin to break down further, and the sugar will fully dissolve.
Simmer the jam: Once the mixture starts to bubble gently, reduce the heat to medium-low and let it simmer uncovered for about 20 to 25 minutes. Stir the jam every few minutes to keep it from sticking, and watch as it gradually thickens and takes on a glossy, jammy consistency. The longer it cooks, the thicker it will become—but remember that it will continue to thicken slightly as it cools.
Optional texture adjustment: If you prefer a smoother jam without chunks of fruit or seeds, you can use an immersion blender to blend the mixture right in the pot. For an extra-smooth texture or to reduce the number of seeds (especially from the blackberries and raspberries), you can strain the jam through a fine mesh sieve after cooking.
Cool and store: Remove the saucepan from the heat and allow the jam to cool slightly in the pot, just until it’s no longer bubbling. Carefully transfer the warm jam to a clean glass jar or airtight container. Let it cool to room temperature, then cover and refrigerate.
Storage: The jam will keep well in the refrigerator for up to 2 weeks. It also freezes beautifully—just leave a little room at the top of your jar to allow for expansion.