Preheat oven to 450 degrees. Pour Guinness into a bowl and set aside so beer flattens.
Rub roast with steak seasoning. In large non- stick pan over medium-high heat, add vegetable oil. Sear roast on each side until browned. Immediately remove meat from pan and set in large roasting pan.
Slice the onions, carrots, and celery then brown them in the same pan you used for browning the roast.
Add the browned vegetables, herbs, and stock to the pan.
Roast covered in preheated oven 1 hour. Reduce heat to 350 degrees and add Guinness. The liquid level should be at least halfway up the side of the roast. Roast an additional 2 hours, check liquid level periodically, and add water if needed.
After 3 hours, the roast should be fork-tender. Transfer roast onto a warm platter. Place roasting pan on stovetop, on high, stirring juices constantly. Make a slurry by combining water and cornstarch. Gradually add to juice until the mixture thickens. Cook 3 to 4 minutes. Remove from heat and serve over meat and vegetables.