This jam has a good, concentrated flavor: The only liquid comes from the rhubarb and berries interacting with sugar and lemon juice. The quick-to-prepare jam also makes a nice filling for tarts.
Combine rhubarb pieces, sugar and fresh lemon juice in heavy large Dutch oven. Cover and refrigerate until juices form, stirring occasionally, at least 8 hours or overnight.
Bring rhubarb mixture to simmer over medium heat, stirring until sugar dissolves. Increase heat to high and boil until rhubarb mixture thickens slightly, about 5 minutes. Add raspberries and boil until mixture is thick, stirring occasionally, about 6 minutes longer. Remove jam from heat. Stir in ground cardamom. Cool jam completely. (Jam can be made 1 week ahead. Store in covered container in refrigerator.)
