There are lots of versions of this sauce, but this one is pretty simple, relying on only a few unusual ingredients: The chile bean paste seems hard-to-find, but it's available anywhere there is an "Asian Section" of a supermarket, and I've seen them in lots of small towns, so just check. As for the plums, use what you have. I have access to lots of wild plums, but domestic plums are fine, as are those cool ornamental plums you see as landscaping trees in warmer climates; they're the deep red-purple trees, leaves and all. The plums are the same color as the leaves. This recipe makes 3 pints.
You will need to pit the plums sooner or later. If you are using domestic, freestone plums, cut them in half and pop out the pit. Most plums, however, have fruit that sticks to the pit. In this case, you can do one of three things: Use a cherry pitter to remove the pits; slice and cook, then strain; or freeze the plums whole to soften, then cook and strain. Freezing loosens the fruit's grip on the pit. Either way you will need some way to strain the pulp from the pits. I use a simple food mill.
I cook the plums in two stages. In the first stage, I put just a little water in the bottom of a heavy pot and add all the plums. Turn the heat to medium and, once they are simmering a little, use a potato masher to squash them as they cook. Keep agitating the plums as you simmer them gently -- you will probably need to turn the heat down at some point -- until most of the flesh comes off the pits. This takes about 20 to 35 minutes.
Move all the plums to a food mill. Pick out as many pits as you can right off the bat. Mill the plums into a coarse puree, removing pits as you go. This is actually really easy with domestic plums, as both the pit and the plum itself is larger than its wild cousin. Wild plums are only about the size of a large cherry, so this stage will take a bit longer with them.
Once the cooked plums are free of pits, return them to the pot along with all the other ingredients. Simmer gently for another 30 minutes, until all the flavors are melded and the garlic, shallot and ginger have softened. Pour everything in to a blender and puree until smooth.
Your plum sauce is now ready, and it will keep for about 6 months in a sealed jar in the fridge. If you want to keep it in the pantry, you will need to water-bath can it. Use clean jars and new lids and process for 15 minutes. My advice is to use half-pint jars, as a little of this sauce goes a long way. Once canned, the plum sauce will keep for a year or more in the pantry.