Trim the fat from the pot roast. Heat oil in a skillet and brown the meat on all sides.
Place the onions and potatoes in the crock pot and place the meat on top.
Combine the remaining ingredients and pour over the top. Cover and cook on low for 8-10 hours or on high for 4-5 hours.
Remove meat and vegetables and place on platter.
If desired, for gravy, pour juices into a glass measuring cup. Skim off fat and add water to equal 2 cups of liquid. Pour into a small saucepan over medium heat. Mix 2 tbs cornstarch and 2 tbs cold water and add to liquid. Cook and stir until thick and bubbly.