Step 1

Toss the raspberries, sugar and lemon zest and juice in a medium saucepan to combine, then mash slightly to release some of the berry juice. Set aside for 10 minutes to allow the raspberries to macerate and the sugar to dissolve. Meanwhile, place a small plate in the freezer to chill; this will be used to test the doneness of the jam.

Step 2

Place the saucepan over medium-high heat. Bring the mixture to a boil and then reduce the heat to medium, stirring frequently. Continue to cook until the jam becomes thick and syrupy, about 25 minutes. To test doneness, place a small amount of the jam onto the chilled plate and freeze for 2 minutes. Drag your finger or spoon through it. If the jam holds its shape and thickness, the jam is done; remove from the heat. If it's runny, continue to cook and test again in 5-minute increments. Once the jam has reached the right consistency, transfer to jars and cool completely before storing, covered tightly. Jam will keep in the refrigerator up to 6 months.