Set a cast iron skillet over high heat, and turn your crockpot to high heat setting. Let both pre-heat for 5-10 minutes, while you prep the meat and vegetables.
While your pan and crockpot pre-heat, pat the roast dry with a paper towel and season generously with kosher salt and black pepper. Set aside and quarter the onions.
Add olive oil to your skillet once it's pre-heated, and then set the roast into the hot pan for about 2 minutes per side.
At the same time, add oil to the crockpot, and let your onions sauté for several minutes as the roast is searing. Stir in tomato paste, and set roast on top of the onion mixture, after it's been seared on all sides. Pour any juices from the skillet into your crockpot as well.
Season once more with salt, pepper, and garlic powder. Add thyme, and spread minced garlic all over the top of the roast.
Pour beef broth into the crockpot, until it reaches just over halfway up the pot roast. Cover, and cook on high for about 4 hours.
After about 4 hours, if your meat has tenderized enough, break it apart with a fork. (if not, no worries!) Add carrots and potatoes, and more broth if desired.
An hour or so before you're ready to eat, make a cornstarch slurry with your cornstarch and water. If you haven't shredded your meat yet, shred it now, and add cornstarch slurry. This will help thicken the broth a bit more. Stir crockpot mixture, cover again, and let it continue to cook on high.
When you're about ready to eat, prepare your egg noodles according to package instructions. Drain them, and add butter. Stir to coat, and serve topped with the pot roast.