01 - Preheat the oven to 400°F (200°C).
02 - Heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery; cook for 6 to 8 minutes until softened.
03 - Stir in garlic and cook for 1 minute. Add ground beef and cook until browned, breaking up meat as it cooks.
04 - Stir in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and black pepper. Cook for 2 minutes.
05 - Sprinkle flour over mixture and cook, stirring, for 1 minute.
06 - Pour in beef stock, stir well, and bring to a simmer. Cook for 10 minutes until thickened. Stir in peas and remove from heat.
07 - Meanwhile, boil potatoes in salted water for 15 to 18 minutes until tender, then drain thoroughly.
08 - Mash potatoes with butter, warm milk, salt, and white pepper until smooth, then fold in grated Cheddar cheese.
09 - Spoon beef filling into a 2-quart ovenproof baking dish. Spread Cheddar mash evenly on top.
10 - Sprinkle 2 tablespoons grated Cheddar cheese over the mash.
11 - Bake for 25 to 30 minutes, or until the top is golden and the filling is bubbling.
12 - Allow to rest for 10 minutes before serving. Garnish with chopped chives or parsley, if desired.