Preheat oven to 375 degrees.
1. Bring large pot of water to a boil, cook potatoes until tender, about 20 minutes; drain.
2. Add milk, butter, nutmeg, salt and pepper to potatoes and mash until smoo th and fluffy.
3. Saute onions, carrot, and celery in oil over medium heat for 5 minutes. Sprinkle with thyme and chili flakes.
4. Add ground beef, and cook until no longer pink.
5. Stir in roast beef, sprinkle with flour, then pour in beef broth, tomato paste,worcestershire ssauce, salt & pepper; stir well. Reduce heat and cook for 5 minutes.
6. Spread meat mixture in a square casserole, spread mashed potatoes over to p and bake 35-45 minutes until heated through.
7. Place under broiler for 1 minute.