Method

• 1.
  Process rhubarb in a food processor to a rough paste (leaving some chunks if desired). Add berries and pulse to break up. Place mixture in a large heavy-based pan with sugar and cook over low heat until fruit has released its juices (don't allow mixture to boil).
• 2.
  When sugar has dissolved, add butter, increase heat to high and bring to a full rolling boil, stirring occasionally. Boil for 4 minutes only.

• 3.
  Remove from heat and skim any foam off the surface. To check for setting, drop a small teaspoons of jam onto a chilled plate. Stand for 1 minute, then run your finger through the jam - if it wrinkles, it's set. If not, return jam to the boil for 1-2 minutes and repeat process. Immediately bottle in sterilised jars.