In a slow cooker (at least 6 quart size), combine onion, garlic, celery and optional leek. (For added flavor, you can sauté the onion, garlic, celery and leek in a skillet until golden brown first, then add to the slow cooker.) Add ham, potatoes, corn, chicken broth and dry mustard. Stir, cover and cook on medium or low for 8 hours. When chowder is cooked, add milk, sour cream and cheese, stirring to combine, and cook for another 15 minutes or until cheese has melted and broth has thickened. (Optional: Stir 1-2 tablespoons of cornstarch into milk before pouring into the soup for a thicker consistency.) Cook on high for 25 minutes or until cheese has melted, stirring occasionally. Add pepper and stir to combine. When done, serve in a bread bowl or soup bowl with additional cheese sprinkled on top. Garnish with chopped fresh parsley.