Rinse beans. In a medium saucepan combine beans and 3 cups water. Cover and let stand 8 hours or overnight. (Or, bring to boiling; reduce heat. Simmer for 2 minutes. Remove from heat. Cover and soak for 1 hour.) Rinse and drain beans.
In a 1-1/2-quart casserole or bean pot combine soaked beans, onion, celery, and carrots. Stir in molasses, salt, mustard, pepper, and 2 cups water.
Bake, covered, in a 325 degree F. oven for 4-1/2 to 5 hours or until beans are tender and most of the liquid is absorbed, stirring occasionally and adding more water as needed (up to 1 cup). Makes 8 servings.