Made beef bourguignon in Le Creuset Dutch oven; can I leave it in this pan in the fridge overnight if I cool it down first to room temp.?

Sure. But first get it cooled down. The cooking vessel is dense, hence it's ability to cook slowly at low temperatures. I keep former personable water bottles, clean and filled 3/4 full with water before freezing to use to chill batches of stock, soup, and even boeuf bourguignon before refrigerating or freezing.

Yes, leave it in the dutch oven overnight, refrigerated. It will taste better.

Not only can you, the flavor will be even better tomorrow. I regularly do braised things in my Le Creuset a day or so ahead (in fact, I have a stracotto cooking right now for a dinner tomorrow.) It's perfect for that.

Thanks

Sure. I often store things in my LC pot until I serve it or portion it out. If you leave the lid off and stir it occasionally, it will cool faster.