Combine all the ingredients in a large soup pot and heat on medium–high. Bring the mixture to a boil.
Mash the fruits with a potato masher and continue simmering the compote on medium heat until it's reduced to about 2 cups. The compote will get very, very thick, and you will be able to run a wooden spoon through it and form a pretty stable groove. It will be thick like tomato paste almost.
Cool overnight in the fridge
Calories calculated based on 1 tbsp (15 mL) serving of rhubarb apple compote