01 - Set the oven to 400°F (200°C).
02 - Heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery and cook 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 browns.
04 - Mix in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and black pepper. Cook for 2 minutes to combine flavors.
05 - Sprinkle flour over the mixture and cook, stirring continuously, for 1 minute.
06 - Pour in beef stock, stir thoroughly, and bring to a simmer. Cook for 10 minutes until thickened. Add frozen peas and remove from heat.
07 - Place potatoes in salted boiling water and cook for 15 to 18 minutes until tender. Drain thoroughly.
08 - Mash potatoes with butter, warm milk, salt, and white pepper until smooth. Fold in grated Cheddar cheese.
09 - Spoon beef filling into a 2-quart ovenproof baking dish. Spread Cheddar mash evenly over the top.
10 - Sprinkle additional grated Cheddar cheese over the mashed potatoes.
11 - Bake in the preheated oven for 25 to 30 minutes 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.