Toss rhubarb with ½ cup brown sugar. Cover and let sit at least overnight, or up to 48 hours. (Refrigerate after 12 hours if not using right away.) It's important to do this, as it causes the rhubarb to "set" as we say when making preserves, i.e., the sugar causes the fruit -- or in this case, the vegetable!! -- to retain its shape and structure.
When the rhubarb is ready, cook the onions in the oil with a pinch of salt over medium heat until just translucent, stirring frequently. Then turn the heat down and cook for another thirty minutes, stirring every four or five minutes, taking care not to let them get too brown.
Add the wine, 1 teaspoon of salt, the Worcestershire sauce, twine vinegar, maple syrup and mace. Cook over medium heat, stirring frequently, for about twenty minutes, until the liquids are thick and gooey. Add the juices that have accumulated from the rhubarb and stir well. Cook for another minute or two.
Stir in the rhubarb and cook for about three minutes. Turn the heat off and cover; let it sit for about ten minutes. Remove the lid, stir well and turn the heat back on. Cook for another minute.
Stir in the mustard, thyme and ground pepper.
Pour into sterilized jars and vacuum seal for long term storage. Or refrigerate for up to four weeks in a jar with a vinegar-proof lid.
Please note: Don't skip the first step of allowing the rhubarb to sit overnight with the sugar. Setting the rhubarb (see Step 1) causes it to retain just the slightest bit of crunch, so like a vegetable and unlike any fruit, which is particularly nice in a condiment of this sort. ;o)
When I'm not working (negotiating transactions for internet companies), or outside enjoying the gorgeous surroundings here in the San Francisco Bay Area, I'm likely to be cooking, shopping for food, planning my next culinary experiment, or researching, voraciously, whatever interests me. In my kitchen, no matter what I am doing -- and I actually don't mind cleaning up -- I am deeply grateful for having the means to create, share with others and eat great food. Life is very good. ;o)