01 - Set the oven temperature to 400°F.
02 - Heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery, sautéing for 6 to 8 minutes until softened.
03 - Stir in minced garlic, cooking for 1 minute before adding ground beef. Brown the beef thoroughly, breaking up any lumps.
04 - Mix in tomato paste, Worcestershire sauce, dried thyme, rosemary, salt, and black pepper. Cook for 2 minutes to blend flavors.
05 - Sprinkle flour evenly over the mixture and stir for 1 minute to remove raw taste.
06 - Pour in beef stock, stir well, bring to a simmer, and cook for 10 minutes until the sauce thickens. Stir in frozen peas, then remove from heat.
07 - Boil potatoes in salted water for 15 to 18 minutes until fork-tender. Drain thoroughly.
08 - Mash potatoes with butter, warmed milk, salt, and white pepper until smooth. Gently fold in grated Cheddar cheese.
09 - Transfer beef filling into a 2-quart ovenproof baking dish. Evenly spread the Cheddar mash over the top.
10 - Sprinkle additional grated Cheddar cheese over the mash.
11 - Bake in the preheated oven for 25 to 30 minutes until the topping turns golden and the filling bubbles.
12 - Allow the dish to rest for 10 minutes before serving. Garnish with chopped chives or parsley if desired.