We the use 5 minute drywall patching compound to fill in the area between the pieces and build the surface up to the same level as the old, surrounding good plaster.
To make sure things stay nice and flat, we use a piece of angle iron (about 4 feet long) that can bridge the damaged area. By using the good plaster as a guide, the excess new compound can be shaved off before it’s completely dry. We found that by getting the surface as close to where it needs to be while the compound is still workable cuts way down on sanding after it’s dry. Also, less dust is a good thing.