Warm up your canning jars and lids in a bath of hot water. Do not boil.
Trim away excess fat from the corned beef. Rinse and chop into 1' pieces.
Boil 6 cups of water in a large pot. (Not the same as the pot the jars are heating in.)
Place the hot jars on a heat-safe trivet. Fill with equal portions of potatoes, onions and corned beef, in that order. If adding salt and pepper, place between potatoes and onions.
Fill each jar with boiling water, leaving 1" headspace.
Remove air bubbles. Wipe jar rims with a clean paper towel.
Place lids onto jars. Tighten rings to just finger-tight.
Place in prepared pressure canner. Process 90 minutes for quarts, 75 minutes if using pints.
Remove after 10 minutes to a dishtowel. Allow to sit for 12 hours. Check lids for proper seal (making sure the lid does not flex) before storing.
