Empty two containers of imPRESS strawberry-blueberry-rhubarb purée and spread evenly.
Place in preheated oven and set a timer for 3 hours. After 3 hours check by lightly touching the middle of the jelly. If it is dry to the touch it is ready. Cooking time will vary depending on how hot the oven is. Cook time could take up to 4 hours.
Remove from oven and allow to cool completely.
The edges of the jelly will be crispier than the middle so using a pizza cutter, cut away the dry crisp edges.