In a large dutch oven or pot (that can go from stove to oven) add butter and heat on stove until butter is melted. While butter is melting, season roast with salt, pepper and garlic salt. Brown roast on all sides. Add the onion slices all over the roast then add the tomato sauce and water. Cook in oven uncovered for about 2 hours or until roast registers cooked using a meat thermometer. Remove roast from pot. Let roast rest for 10 to 15 minutes. While the roast is resting, take the pot and place on stove, add flour to juices and cook-stirring continually to make a gravy.
***note-- 20 minutes per pound of roast is the average cooking time**