Wipe pot roast with paper towels. Heat oil in Dutch oven and add onions. When they are golden, remove onions and brown roast on all sides, about 10 minutes. Return onions to pot and add peppers, garlic, broth, salt, and pepper. Reduce heat and simmer covered for about 3 hours or until tender. Stir occasionally, and add a little water if more liquid is necessary.
When meat is done, remove from pot. Let rest for 10 minutes and then slice.
Meanwhile, place sauce in blender for a few seconds. It should be fairly thick, but if not, mix a little flour with some of the juice from the sliced meat and add to sauce.
Return meat and gravy to Dutch oven and let simmer for about 20 minutes before serving.