1. In a large saucepan, mix blueberries with sugar and lemon juice. Set aside at room temperature for 3 hours. This will help to create a lot of juice. Then, simmer blueberries and sumac together over medium-low heat, stirring occasionally with a wooden spoon and skimming foam as it rises, until syrup is thick and berries are soft (crush berries with the back of a spoon), about 15 minutes. Remove from heat and cool for 5–10 minutes.
2. Sterilize four 1-pt. mason jars by plunging jars, lids, and rims into a pot of boiling water. Drain and set aside to dry. Pour jam into jars and let stand at room temperature to set. (Store in the refrigerator for up to 1 month.)
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I love this jam on toast with poached egg and shaved manchego cheese. Yum!