Melt butter in a dutch oven over medium heat. Add chopped onion and sauté 8 minutes or until tender. Stir in the cream of potato soup, milk, and pepper, bring to a boil. Add shrimp, reduce heat and simmer stirring often for 5 minutes or just until the shrimp turn pink. Stir in cheese until melted. Garnish with chopped parsley if desired. Serve immediately with the oyster crackers. Hint: Add a small amount of instant potato buds to thicken the chowder after the shrimp are fully cooked. Makes about 12 cups.