Shell and wash peas. Measure 8 cups of peas. Wash and peel onions. Measure 3 cups of onions.
In a 6- to 8-quart Dutch oven or kettle heat water to boiling; add peas and onions. Return to boiling. Remove from heat.
Using a slotted spoon, pack hot vegetables loosely into hot, clean pint canning jars, leaving 1-inch headspace. If desired, add 1/4 teaspoon salt to each jar. Add the boiling cooking liquid to each jar, leaving 1-inch headspace. Remove the air bubbles, wipe jar rims, and adjust lids. Process the filled jars in a pressure canner, at 10 pounds pressure for weighted canners or 11 pounds for dial-gauge canners, for 40 minutes. Allow the pressure to come down naturally. Remove jars from canner and cool on racks.