Turn pressure cooker to the "saute" setting. Melt butter and saute onion, celery, and carrots, seasoned with salt and pepper, for about 5 minutes, until softened.
Add the split peas, stock/water, and ham.
Cover pressure cooker and set to manual high pressure for 15 minutes.
Allow the pressure to naturally release for 10-15 minutes.
If you used a ham bone: remove the it from the soup and remove the ham meat that's still attached to it.
Add the ham meat back into the soup.
Taste and adjust seasonings if necessary. Serve immediately, or store in your refrigerator for up to 1 week
Serve immediately, or store in your refrigerator for up to 1 week or freezer in an airtight container for up to 6 months.