Preheat oven to 400 F and spray a casserole dish (about 9x13 sized) with non stick cooking spray.
In a medium pan, cook carrots and onions in olive oil over medium high heat. Cook vegetables until soft, but not caramelized, about 5-10 minutes. While vegetables cook, combine beef stock, corn starch, thyme, Worcestershire sauce, and Dijon in a small bowl and set aside.
Layer corned beef in prepared pan. Add a layer of vegetables and then pour sauce evenly over the vegetables. Top with an even layer of mashed potatoes. Place in oven and cook 20-30 minutes or until everything is heated through.