Prep for testing the jam set: put a couple of small plates in your freezer before making the jam.
Instructions
Place berries in a large, wide saucepan.* Mash them for 3 to 4 minutes over medium heat. (A potato masher works really well for this).
Add the lemon zest, lemon juice, and sugar, and stir over medium heat until sugar is dissolved.* Increase the heat and bring to a boil and cook steadily for about 5 minutes, stirring frequently. (Using a flat-bottomed spoon or spatula can help prevent scorching.)
When your jam looks like it's starting to gel (or is at around 215° degrees if using a candy thermometer), take it off the heat and remove a plate from the freezer.* Drop a teaspoon of hot jam on it and let it stand for a minute then push the blob of jam with your finger. If the surface of the jam wrinkles even a little bit then it has set; if it is still quite liquid, put the pan back on the heat and boil the jam for another 3 to 5 minutes before testing again.
Once jam is set, skim and pour into clean and sterile jars. Seal the jars and place them where they can cool undisturbed and continue setting for 8 hours. Store in the fridge.*
Notes
*The volume increases a lot so choose your pan accordingly. The width of the pan matters, too, as a wider pan helps the liquid to evaporate and the jam to set more quickly.*Warming your sugar ahead of time can help it dissolve faster. Place sugar in a shallow oven-safe pan and warm in a 250°F oven for 15 minutes before adding it to the fruit.*If using a candy thermometer, around 220°F is the setting point for jam.*This recipe is based on small batches of jam meant to be refrigerated and eaten quickly. If you're planning on longer-term cupboard storage, you may want to follow more stringent sterilization and canning procedures, such as those found here.