Preheat oven to 300 degrees.
Pat the roast dry with a paper towel. Season both sides evenly with the Montreal Steak Seasoning. In a large Dutch oven, heat the olive oil over medium-high heat. Brown the meat on all sides. Remove the meat from the pan to a plate. Set aside.
Add the onions, salt and pepper to the pan and cook over medium heat until tender, about 7-8 minutes. Add the garlic, thyme leaves and bay leaf and continue to cook for one more minute.
Add the red wine and cook, stirring up any brown bits from the bottom of the pan. Return the roast to the pan and any accumulated juices from the plate. Add the beef broth, then add the celery, carrots and potatoes to the pan, arranging around the roast.
Bake, covered until the meat is fork tender, about 2-2 1/2 hours.
Remove the roast and vegetables from the pan to a plate. Keep warm. Discard the bay leaf from the pan and skim off any fat. Bring the juices to a boil, then reduce the heat and simmer for 5 minutes to reduce just slightly.
Remove a few spoonfuls of the liquid to a small bowl. Whisk in the 3 tablespoons of flour until smooth. Add the flour mixture to the liquid in the pan and whisk until thickened.
Drizzle desired amount of gravy over sliced pot roast. Serve remaining gravy on the side.