01 - Set the oven to 400°F.
02 - Heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery; sauté for 6 to 8 minutes until softened.
03 - Stir in minced garlic and cook for 1 minute. Add ground beef and cook until browned, breaking it up as it cooks.
04 - Mix in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and black pepper; cook for 2 minutes.
05 - Sprinkle flour over the mixture and stir for 1 minute. Pour in beef stock, combine well, and simmer for 10 minutes until thickened.
06 - Fold in frozen peas and remove from heat.
07 - Place potatoes in salted water and boil for 15 to 18 minutes until 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. Spread the Cheddar mash evenly over the top.
10 - Sprinkle remaining grated Cheddar cheese over the mashed potatoes.
11 - Bake in the preheated oven for 25 to 30 minutes, until the topping is golden and the filling is bubbling.
12 - Allow the dish to rest for 10 minutes before serving. Garnish with chopped chives or parsley, if desired.