1. Turn pressure cooker to the "sauté" setting. Melt butter and sauté onion, celery, and carrots, seasoned with salt and pepper, for about 5 minutes, until softened.
2. Add the split peas, stock/water, ham, and bay leaves.
3. Cover pressure cooker and set to manual high pressure for 15 minutes.
4. Allow the pressure to naturally release for 10-15 minutes.
5. 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.
6. Taste and adjust seasonings if necessary. Serve immediately, or store in your refrigerator for up to 1 week or freezer in an airtight container for up to 6 months.

Notes:

1. For a vegetarian version, omit the ham, use more salt and pepper, and make sure to use vegetable stock or water instead of chicken. For a vegan version, use olive oil instead of butter.
2. To make this in your slow cooker, add all ingredients (omitting the butter) to slow cooker and cook on low for 8 hours or high for 4 hours, or until split peas are cooked to desired tenderness.
3. To make this on your stovetop, sauté veggies as instructed in a large pot. After adding the remaining ingredients, bring to a boil, cover, and simmer for approximately one hour (or until split peas are cooked to desired tenderness).
4. This soup will likely thicken once it cools and is refrigerated, especially if you use a ham bone to make it because of the gelatin. Thin out with water and adjust seasonings as necessary.