Plum “Mostarda”

I’ll admit defeat here and put quotation marks around the noun. Without true mustard essence, which isn’t available in the United States because it’s toxic in large doses (as, of course, are many other grocery items that are nonetheless readily accessible here), one can’t make a real mostarda. I should note that what I’m attempting to reproduce is not the whole mixed fruits candied in a clear mustard-flavored syrup—mostarda di Cremona—but the more rustic, thick, jamlike mostarda di M