01 - Preheat oven to 400°F (200°C).
02 - Bring a large pot of salted water to a boil. Add potatoes and cook for 15-20 minutes until fork-tender.
03 - While potatoes cook, heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery; sauté for 5-6 minutes until softened.
04 - Add garlic and cook 1 minute. Add ground beef, breaking it up as it browns, about 6-8 minutes. Drain excess fat if needed.
05 - Stir in tomato paste, Worcestershire sauce, thyme, and rosemary. Cook for 2 minutes.
06 - Pour in beef broth, bring to a simmer, and cook for 10 minutes until the mixture thickens slightly. Stir in peas. Season with salt and pepper. Remove from heat.
07 - Drain potatoes and return to the pot. Add butter and milk; mash until smooth. Season with salt and white pepper.
08 - Spread beef mixture evenly in a 9x13 inch baking dish. Top with mashed potatoes, spreading to cover filling completely.
09 - Use a fork to create ridges in the potato topping for crispiness.
10 - Bake 25-30 minutes until the top is golden and filling is bubbling. Let rest for 10 minutes before serving.