Preheat oven to 375 degrees. Place roast in a roasting pan that has a lid.
Sprinkle with the seasoning salt. Arrange onion slices on top. Then evenly top with dry soup mix. Pour water around the roast. Cover.
Cook for two to two and a half hours or till tender. Add vegetables for last half hour of cooking time. They should be fork tender when all is done. Be sure to baste roast occasionally while it cooks with juices.
Remove roast and vegetables to a platter to rest before slicing to keep juices from running out. While it is resting, you will make the gravy from the juices left in the pan.
Take an empty peanut butter jar or jelly jar and place one inch of flour in the bottom. Add the cornstarch and salt and pepper to taste. Tighten lid. Shake to combine. Add water enough to fill to halfway point and shake again to combine.
Place roasting pan with meat juices on stove top over low heat. Pour in contents of jar and cook and stir till thickened scraping up particles from bottom of pan as you go. Serve gravy in a gravy boat or in bowl with serving spoon alongside meat and vegetables.