1. In large heavy skillet, brown both sides of chuck roast in hot oil over medium high heat.
2. When browned, turn off heat, remove meat to plate and set aside.
3. In same skillet with heat off, mix in the green onion, cranberry sauce, water, salt, and pepper, stirring until mixed well and the brown bits on pan are loosened; then spoon mixture into crockpot.
4. Put small rack in crockpot and place browned meat on rack.
5. Put cover on and set crockpot to LOW and cook for 7 to 8 hours.
6. When meat is done cooking, remove meat and rack from crockpot; put meat on serving platter and keep warm.
7. In small bowl, mix together the cold water and cornstarch until completely dissolved.
8. Slowly drizzle the cornstarch into the liquid in crockpot, stirring constantly; continue stirring until mixture thickens and forms gravy.
9. Serve the gravy over mashed potatoes alongside the beef roast.