Simple and simply delicious! If you have leftover cooked chicken, you can use that instead. Just stir it in with the peas and onions.
If sweet peas are in season at your farmers market (or if you're lucky enough to have a garden) then go ahead and use fresh peas. They should be partially cooked, though, before you add them to the soup. I just boil some water, dump in the peas, turn off the water, and let them sit for about 1 minute. Drain them and they'll be lightly blanched, ready to add to the soup. The 2 minutes cooking time won't make them mushy as long as you don't overcook them during the blanching or steaming process. You could just throw them into the soup without blanching them, but then simmer a few minutes longer.