Microwave sparking mystery---solved?

I posted about a year or so ago about putting some king mackerel in the microwave and having it put on a display of sparkling like it was covered in aluminum foil. I couldn't figure out what caused it---maybe it had accumulated heavy metal in the meat. It was kind of scary.

Tonight, it happened again, but this time with onions. I had just chopped them up and wanted to sort of saute them, but they started giving me a fireworks display. Okay, this time I was all over it and realized that I had just sharpened the chef's knife on my steel. Obviously, there were little particles left in the onions. Mystery solved.

Skill and science wins out over fear and superstition everytime...

Always clean and wash after sharpening to remove the Metal shavings they can cause internal bleeding. Just a note.