Brown both sides of roast, well. Slice onion and lay across top of roast. Reduce heat and add 2 cups water. Cover and simmer for 45 minutes. Add more water as needed. Peel carrots and potatoes, cut potatoes in half, and carrots to fit (you can use a small bag of baby carrots). Add to simmering roast and cook until potatoes and carrots are soft. Remove meat and vegetables from pan and keep warm.
Make gravy: Mix 1 cup water and 1 1/2 T flour in jar. Add to pan and drippings, stir well. Add 1 to 2 cups more water and salt and pepper to taste. Serve.