1: Put 500 g rhubarb in a pan, after preparing it as shown here. Tip 150 g caster sugar over the top and shake or stir gently.
2: Leave at room temperature for the sugar to act, and after 30 minutes or so you'll see that rhubarb juice has begun to collect in the bottom of the pan.
3: This is the sign that you can put it on to cook gently, covered with a lid.
4: Leave to cook on low heat for about 20 minutes until it is "stewed". Check by stirring: you should no longer be able to see individual pieces of rhubarb. They should have disintegrated into a smooth purée. Your stewed rhubarb is ready. Leave to cool, then keep refrigerated in a sealed container.
Remarks
Taste the fruit before it's completely cooked to see if it needs a little more sugar. If you'd like a smoother result, you can blend the rhubarb briefly, but it's hardly necessary.