Preheat oven to 425°F.
Place potatoes in a pot of cold salted water and bring to a boil. Cook for 15 min or until tender when pierced with a fork. Drain. Return potatoes to a pot over low heat for 1 min to evaporate any remaining water. Add 2 tbsp butter and mash until combined and fluffy. Add milk to loosen, season with 1⁄۴ tsp salt and stir in half of cheese. Set aside.
Heat remaining 2 tbsp butter in large frying pan over medium. Add carrot, onion and celery and sauté for 5 min or until beginning to soften.Turn heat to high. Add beef and garlic, and sauté, breaking up meat with a wooden spoon, for 5 min or until meat begins to brown. Add flour, stirring for 2 min to cook out floury taste.
Add stock, ketchup, mustard and thyme. Bring to a boil and simmer for 3 min or until broth has reduced and thickened and made the meat saucy. Season with remaining 1⁄۲ tsp salt and pepper and stir in peas.
Transfer beef mixture to a deep 9-in. ovenproof casserole dish. Spread mashed potatoes overtop. Sprinkle with remaining cheese. Bake for 15 min or until sauce is bubbling. Turn to broil and cook until potatoes are golden.