Season the check roast with the kosher salt, pepper and thyme.
Heat you pan.
Add the canola oil and when it ripples and is hot add in the roast and brown, deeply, for 4-5 minutes on each side. ( Less for smaller roast, and I try to also brown edges in addition to sides). Brown to your liking.
In your slow cooker add the carrots, potatoes, garlic, celery and onion.
Lay the beef on top, then add the beef broth and cover, cooking on low for 8-10 hours or 5-6 on high. (I think low is much better, more tender).
In the last hour mix your cornstarch and water and add it to slow cooker to thicken the sauce or you can take the food out when done cooking and add the leftover liquid to a small saucepan with the cornstarch / water mixture and cook on high for just 2- 3 minutes until the liquid is thickened into gravy.
Pour the gravy over the meat and garnish with parsley if desired.