In a 3-quart pan or Dutch oven, heat olive oil over medium-high heat until shimmering. Sauté carrots, celery, and onion until softened and translucent, about 5 minutes.
Stir in broth, water, lentils, ham bone, garlic powder, cumin, paprika, thyme, and bay leaf. Bring to a boil. Pour into slow cooker. Heat on HIGH for 3 to 4 hours, LOW for 6 to 8 hours, or until lentils are tender.
Remove ham bone and bay leaf. Chop ham from bone and return to slow cooker. Add additional leftover ham, if using, and cover until heated through, about 5 minutes. Season to taste with salt and pepper and garnish with parsley if desired.