...would be a great name for a rock band.

Okay, maybe not so much.

LW, if you aren't confrontational but you can do passive-aggressive, I suggest bringing your mustard in packets for a while and secretly mixing laxatives, Ipecac or something nasty-tasting into the jar. ;)

In all seriousness, I suspect that this woman is using your mustard right in front of you - in spite of your initials on it - because she knows you're too shy to confront her and so she can get away with it. I've had this kind of thing happen to me.

You can do what other posters have suggested and keep your mustard in your office, since it won't go bad. Or you can just go up to the woman and politely inform her that the mustard with your initials on it is for your personal use. Make sure you do this in a public place and have a friend nearby to back you up so the mustard thief doesn't feel like she can make a scene.