Preheat a pan over high heat. Turn on the vent.
Pat chuck roast dry and season on all sides with salt and pepper then cover in the oil on all sides rubbing it in so the whole surface is covered but not dripping.
Set a crockpot to low and add cream of mushroom soup, beef broth, red wine, dry onion soup mix, garlic powder and beef base. Mix well.
Sear the chuck roast on all sides for 2 minutes per side including the sides. You will have to use tongs to hold the chuck roast in place to get the sides browned.
Place browned chuck roast in the crockpot. Push the roast to the bottom of the crockpot so it's almost fully submerged in the sauce. Put the onion in the crockpot. Cover and cook for 8 hours.
Remove the chuck roast from the crockpot. It will come off in hunks most likely.
Shred the beef and separate the fat from the meat with two forks. This can take a little bit of time. Discard the fat.
Skim the fat off the top of the sauce in the crockpot.
Add 2 tbsp of the liquid to cornstarch. Mix until fully combined and add to the crockpot. Mix well and allow to thicken for 5 minutes with the lid on.
Add shredded meat back to the crockpot stir and serve.