In a large pot, heat oil over medium-high heat. Add onion, carrots, celery, and salt and pepper. Go easy on the salt, remembering that you will be adding ham later. Cook until onions are translucent and vegetables begin to soften. Add in rinsed peas, water (start with four cups and add more as needed), and bay leaf. Stir to combine and bring to a boil. Reduce heat to medium-low (a simmer), and simmer partially covered for 30-40 minutes until peas are softened. Add in ham and parsley, stir to combine, heat until warmed though. (Note: I add the ham at the end because I find that it gets tough if I have it simmering the entire time. If you have a ham BONE, that is an entirely different story, throw that in at the beginning and let the flavors meld.) Remove bay leaf prior to serving.