Are you interested on how to do this?
Is it 'on' or 'in'?
Yes, you're right. It sounds like 'on' does not collocate with 'interested'.
Whats on your mind. (let's talk about it) < What are you thinking about.
It's all in your mind. < It's only what you think, and nothing else.
What do you have in mind? < What are you planning? What is your plan?
What's in your mind? What sort of thoughts inhabit your mind? Who are you?
It could also mean this:
Whats in your mind. < What sort of evil or bad things do you think about. (not thinking - This would refer to how someone is, not temporary behavior or a temporary attitude.)
you should use the phrase "to be interested in"
i'm not a teacher