In a large dutch oven, heat the oil over medium high heat. Blot the meat dry with a paper towel. When the oil is shimmering, add the beef. Cook the beef until the beef can easily be removed from the dutch oven - about 5 minutes. Turn the cubes until they are browned on both sides and remove to a sheet pan. Reduce the heat to medium and add the sliced onions, carrots and celery to the dutch oven. Cook for about 5 minutes until the onions are soft but not browned. Add the tomato paste and cook until the tomato paste is no longer red but a mahogany color - about 3 minutes. Sprinkle the flour over the mixture and stir until the flour is incorporated - about 3 minutes. Add the guinness, the beef stock, dijon mustard, worcestershire, thyme and both peppers and stir to combine. Cover and cook over medium heat for 2 - 2/12 hours or until the beef is fork tender. Taste and adjust seasonings.