How to reduce a sauce at the bottom of a baking dish?
WOW...this is a published recipe?? Are Pyrex casserole dishes safe for use on electric stovetops?  It is very dangerous to put most bakeware on most stoves. Your best option would be to either originally bake in a stove-safe implement, or to transfer from a casserole to a saucepan at that point.

Only, only use cookware labeled as safe for a stove (usually only metal) on a stove! Period! To do otherwise as asking for potentially deadly shrapnel. I am not kidding!