Peel the rhubarb and cut into small cubes. I like to do this for jam with household scissors - it`s quick.
Peel the apple, cut into eighths and cut into very fine flakes.
Put the sliced oyster in a plastic bowl with a lid and mix well with the preserving sugar, cinnamon, vanilla powder, rum and lemon juice and cover and let rest for about 5-6 hours so that the rhubarb can infuse well.
Then pour the fruit mixture into a tall saucepan and - only now add a pinch of baking soda. The baking soda rounds off the taste of strongly sour fruits such as rhubarb and makes it more digestible.
Bring everything to a boil and cook for at least 4 minutes. Make a gel test.
Then immediately pour the jam into jars that have been rinsed with hot water.