This was a relish my mother used with meat dishes, and sometimes ate it on bread. My older half brother said my father insisted on relish or jelly with the supper meats. This one was easy for her to make, and tastes great with any meat. We had three large rhubarb plants so every spring this was made and bottled for the next winter.
Chop and measure the rhubarb and onions into a jam kettle or large pot, add the sugar and spices and stir, while heating on medium heat. The rhubarb will slowly become very cooked and fall apart, and the mixture will need to be cooked down.
The longest part of the whole thing is the cooking down of the relish, you need to stir almost constantly in a figure 8 motion to keep the relish from scotching, and it takes a while to before it thickens up.
I never do this if I don't have a whole afternoon to devote to it, and a nice flat bottomed not too deep dutch oven is the best to use. I have only doubled it once as that took about 6 hours to get to thickened for bottling. The smaller kettle is better, I have a half sized jam kettle, which has a large flat bottom and it works better than a dutch oven, but its a long slow process.
You can test consistency by dropping some of the relish onto a cold plate, when it holds it shape without running or pooling moisture around the blob, its thick enough to bottle. Place into jelly jars that have been sterilized an heated and top with lids and rings that have been kept hot. These don't need processing as I have never processed jams or jellies only used snap lids and rings and I have never had any go bad.