In a large bowl, add the beef, flour, salt, and pepper, and mix until the beef is well coated in flour. Set aside.
Heat the canola oil in a large pot (at least 6 quarts) over medium-high heat for about 2 minutes.
Add the chopped onion, and cook until translucent, about 3 minutes, then add the garlic and cook until fragrant, less than 1 minute.
Pour in the beef, and cook until brown, stirring occasionally to make sure all sides of the beef are browned. This will take about 10 minutes.
Pour in the Irish Stout, and boil with the beef for 5 minutes, then stir in the tomato paste and thyme and pour in the beef stock.
Add the carrots and potatoes, bring to a boil, then let the stew simmer for 30 minutes or until the potatoes and carrots are tender. Remove from heat.
Scoop out a 1/4 cup of the stew broth into a small bowl, and whisk in the cornstarch to make a stew slurry, then stir the slurry back into the stew.
Serve and top with parsley, if desired.