Mix port, cream of mushroom soup, and tomato paste together, sit aside
Place roast on a plate and apply salt and pepper on all sides.
Cut onion on a mandolin, forming rings. Break up rings and place in slow-cooker.
Place roast on top of onions.
Slice carrots on mandolin, in about quarter size. Set aside.
Cut up potatoes into dice-size cubes. Place potatoes on top of the roast in the slow cooker.
Add mushroom soup mixture.
Place lid on slow cooker and set to high for one hour.
After one hour of cooking, turn slow cooker to low and pour carrots on top of everything.
Replace lid and continue to cook for an additional 6 -8 hours on low.
Remove potatoes and carrots from the pot, placing them in a serving bowl.
Remove roast from slow cooker and using two forks, shred. Place in a serving dish.
Place cornstarch in a large measuring cup. Add about 1/2 cup of remaining gravy from the slow cooker to the cornstarch. Whisk together until no white color remains and the cornstarch is completely incorporated.
Pour mixture back into the slow cooker. Whisk together. Pour gravy into a serving dish.