Balsamic Rhubarb and Onion Ketchup

Directions

1. In a heavy pot with a lid, 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 brown. They should be the slightest golden brown, very soft and sweetly fragrant.
2. Add the wine, brown sugar, 1 teaspoon of salt, the Worcestershire sauce, wine vinegar, mace and allspice. Cook over medium heat, stirring frequently, for about twenty minutes, until very thick and gooey.
3. Add the rhubarb and cook for three or four minutes, until just tender. Turn the heat off and cover; let it sit for about ten minutes. Remove the lid, stir well and cook for another minute, stirring constantly.
4. Add the mustard and thyme leaves; let cool in the pan for about 15 minutes.
5. Puree in the blender until smooth. Add the strawberry balsamic vinegar and blend for just a few seconds to combine. If it tastes too sharp (vinegary), add a tablespoon or two -- one at a time, tasting after the first -- of raw honey.
6. Add the ground pepper and blend to combine.
7. If you plan to vacuum seal for longer term storage, you should put the ketchup back into the pot and heat it to boiling before pouring into sterilized jars and sealing according to the manufacturer's instructions. Or skip the re-heating step and pour into any jar with a vinegar-proof lid and refrigerate for up to four weeks.
8. Enjoy!
9. I posted here on food52 a recipe for strawberry balsamic vinegar. If you don’t have any strawberry balsamic vinegar, or don't have time to make it (as it really needs at least three or four days before it's worth using), use regular balsamic vinegar, and drop into the pot, with the mustard, a handful of strawberries that you’ve coarsely chopped.