01 - Preheat the oven to 400°F.
02 - Place the peeled and chunked potatoes into a large pot filled with salted water. Bring to a boil, then reduce heat and simmer for 15 to 20 minutes until tender.
03 - In a large skillet over medium heat, warm olive oil. Add onion, carrots, and celery, and sauté for 5 minutes until softened.
04 - Stir in minced garlic and cook for 1 minute. Add ground beef, breaking it apart with a spoon, and cook until browned and no longer pink.
05 - Drain excess fat if necessary. Mix in tomato paste, dried thyme, dried rosemary, and flour, cooking for 2 minutes to combine.
06 - Pour in Worcestershire sauce and beef broth. Simmer for 5 to 7 minutes until the mixture thickens.
07 - Stir in frozen peas and season the filling with salt and pepper. Remove from heat.
08 - Drain potatoes thoroughly and return them to the pot. Mash with unsalted butter and warmed milk until smooth. Season with salt and pepper.
09 - Spread the beef filling evenly in a 9x13 inch baking dish. Top with mashed potatoes, smoothing the surface.
10 - Create ridges on the mashed potatoes using a fork for texture. Bake for 25 to 30 minutes until the topping is golden and the filling bubbles.
11 - Allow the dish to rest for 10 minutes before serving.