Place the cranberries, quartered strawberries, lemon juice, and granulated sugar in a small saucepan.
Using a potato masher, gently crush the fruit mixture. Break down some of the berries while leaving some texture intact so the ingredients begin to blend together.
Set the saucepan over medium heat and bring the mashed fruit mixture to a boil.
After it reaches a boil, lower the heat and allow the mixture to simmer for 15–20 minutes, stirring from time to time. Continue cooking until the fruit mixture has thickened enough to coat the back of a spoon and has developed a jam-like consistency.
During the final few minutes, monitor the texture carefully and remove the pan from the heat as soon as the desired thickness is reached to preserve the fresh, vibrant character of the fruit.
Take the saucepan off the heat and allow the Christmas jam to cool completely.
Once fully cooled, spoon the jam into a clean glass jar and refrigerate. Chilling allows the flavors to develop further while maintaining the thick, spreadable texture.