In a bowl combine the mushroom soup with beef broth, minced onion, beef bouillon, onion powder, crushed celery seed, sugar, fresh garlic, and Worcestershire sauce. Mix until combined and set aside.
In a small bowl or large plastic baggie, mix together flour with salt and black pepper. Dredge the roast in the flour/black pepper mixture.
In a medium skillet over medium heat, heat the oil. When the oil begins to ripple, carefully add in the roast and pan sear about 5-6 minutes per side, or until all sides are caramel-colored.
Transfer the roast to the slow cooker, top with mushrooms, and slowly pour the soup mixture over top. Cover and cook on LOW for 8 hours or until the roast is tender.
Remove the roast from the crockpot and place it on a plate. Transfer the sauce from the crockpot into a medium saucepan and bring to a boil over medium heat.
In a small container mix together the milk and cornstarch to make a slurry.
Once the mixture starts to boil, pour in the slurry and whisk until thickened. If desired, add in the Gravy Master. Remove from heat and ladle over the sliced eye of round.