Put the onion and potatoes in a large pot over medium high heat. Cook for 5 minutes. Add the water and liquid from the clams. Cook for another 5 minutes then remove from heat and set aside.
In another cooking pot, melt the butter. Add all-purpose flour, whisk until smooth. Slowly pour cream, a little at a time and stir constantly. Cook for 5 minutes.
Add vegetables and clam juice mixture into the cooking pot. Simmer for 5 minutes over low heat.
Cut the top of the sourdough bread. Remove some of the inside to form a bowl and then pour the clam chowder. Enjoy!