Pot Roast with Cider

I love to make this because it's easy, but I add soy sauce instead of molasses because I don't like sweet flavor and meat.
6 lbs beef pot roast
4 large onions, quartered
4 large celery stalks, including leaves
2 cups apple cider
2 tbs light molasses or brown sugar
2 tsp. salt
6 whole allspice
4 whole black peppercorns
1/2 tsp ground ginger
1/4 c butter or margarine
3 tbs. flour
6 tbs. cold water

Combine cider, molasses and seasoning. Put beef in large glass bowl with onions and celery. Pour cider mixture over beef. Cover and let beef stand for 24 hours, turning at least once so all sides are marinated.
When ready to cook, remove beef and pat with paper towels. Melt butter in heavy dutch oven and brown roast well on both sides. Add marinade and vegetables out of meat juice. Stir in flour and cold water mixture until smooth. Cook stirring constantly until mixture thickens and boils.