Cut the rhubarb into approx ⅓-1/2 inch slices. Finely dice the onion and mince the ginger.
Put everything in a wide non-reactive pot and bring to a simmer. Simmer for around 10 minutes until the mixture thickens, stirring occasionally (but taking care not to break up all the chunks). The liquid should not immediately close in behind the spatula when you move it through the mixture.
Set aside to cool. Store on the refrigerator for a day or two until needed, bringing to room temperature to use.
Notes
This makes approx ¾ cup of chutney - you can easily increase the recipe by multiplying up all of the ingredients. With a larger quantity it may take slightly longer to simmer and thicken.