What's the best way to keep cover of a pan slightly opened?
Tilt the lid.
It will stay where you put it… approximately.
 See pan top left &amp; pyrex bottom right; they will stay like that all day if needed

If it really refuses to stay, then wedge a spatula [or any other bit of wood or plastic (&amp; of course, not metal if it's going in the microwave) of any appropriate size]* in it, either from the edge, down the handle, as at bottom left, or right the way across then balance the lid on top of that, as at top right. The latter will also go in a microwave.



*This is physics 101 - put something in that's big enough to hold the lid ajar, yet not big enough to get in the way of anything else &amp; not small enough to fall in &amp; get lost in whatever you're cooking…