Heat oil in a large frying pan over medium heat. Add ham and cook, stirring occasionally until browned slightly.
Add the onion, leeks, garlic, carrot, and celery and season with black pepper. Cook, stirring occasionally, until tender and translucent, about 5 minutes.
Transfer everything to the slow cooker and add the 1 pound rinsed split peas, the 1/2 pound potatoes, the 8 cups of chicken broth, the 2 bay leaves, the 1 tablespoon kosher salt, and the 1/2 teaspoon ground black pepper.
Cover and set slow cooker to high for 4 hours or low for 6.
Taste and add more salt and pepper if needed. Stir in the parsley and squeeze the lemon over the soup. Stir and taste one more time adding more salt, pepper, or lemon.