Drain oysters reserving juices. Melt butter, sauté onions for 3 minutes until tender. Stir in flour and cook over low heat about 2 minutes stirring constantly. Slowly whisk in oyster juices, beef broth, and water. Cook while stirring until liquid comes to a boil. Season to taste with salt and pepper. Just before serving, return to a boil, add half & half and oysters. Cook briefly just until oysters curl. Adjust seasoning. Serve at once.