For topping, place potatoes in large saucepan of cold, salted water. Bring to boil and simmer partially covered for 20 minutes or until very tender. Drain. Mash in pot until smooth. Add cream, butter, egg yolks, nutmeg, salt and pepper to taste. Mash until smooth and fluffy. Keep warm. For filling, heat 1 tb. oil and butter in large skillet over medium-high heat. Add onion, garlic and chili flakes; cook 2 minutes. Add carrot, celery, thyme, salt and pepper to taste; cook 3 minutes. Set aside in bowl. Add 1 tb. oil to skillet and heat over medium-high heat. Add beef and lamb; cook, stirring continuously, until meat is no longer pink. Add salt and pepper to taste. Discard sprig thyme and any excess fat from pan. Add vegetable mixture to meat in pan. Stir in stock and tomato paste. Reduce heat and cook covered on low heat 15 minutes, stirring frequently. Meanwhile, in separate small frying pan, heat remaining oil over medium-high heat. Add mushrooms; cook 2 minutes until soft. Sprinkle mushrooms on to bottom of ovenproof casserole dish, about 20 cm square. Pour meat mixture over top, spreading evenly. Place tomato slices on meat. Either pipe mashed potatoes over meat mixture using piping bag fitted with star tip or spread potatoes over mixture. Bake in preheated 190ºC oven 25 to 30 minutes until potatoes are golden and meat mixture is hot. If potatoes are not golden, place under broiler 2 minutes. Let stand 5 minutes before serving.