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 1⁄4 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, Worcestershire, 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 1⁄2 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.
Serve and enjoy!