Pour leftover juices from pot roast into gravy seperator if you have a lot left. Drain broth into small pan using as little of the grease that you can. Using a spoon dip onion and other spices left from roast from bottom of seperator. Discard remaining grease.
If your juices are thicker use what is left it will dilute with rest of mixture.
Add half & half,pimento,salt,pepper and broth to mixture. Combine cornstarch with small amount of water just utill smooth. Add the rest of, or enough water for a small pan of gravy taste as you add water to make sure not to get broth too weak,or depending on how much broth you had left from roast.
Stir while cooking over medium heat until boiling. Cook to desired thickness.
Don't boil too long.cornstarch will break down and go back to being thin if over boiled.