Use this expression to talk about a topic that someone thinks about a lot. Aside from food, here are some of the most common topics that people's minds are "always on":
His mind is always on work.
Your mind is always on sex.
Her mind was always on other things.
Most of the time, when you say that someone's mind is "always on" something, it's a complaint. You want to say that they think about that topic too much.
You can also use the phrase "get your mind off of ___":
It's nice to take some time off and get your mind off of work for a few days.