
In a medium bowl toss the cauliflower, onion, chiles and mustard seeds. Transfer to a clean, quart-size mason jar, The ingredients should come nearly to the top (see photos).
In a saucepan over medium heat combine the vinegar, water, salt and sugar. Bring to a simmer. Cook for a few minutes, stirring a bit, until the sugar dissolves.
Carefully pour the hot vinegar into the mason jar, over the cauliflower. Fill to within about 1/2-inch of the top. Close the jar, allow to cool, and refrigerate. Use within a week.