In a large pot, bring salted water to a boil and add the cubed potatoes.

Cook until tender, about 15-20 minutes.

Drain the potatoes and return them to the pot. Add milk, butter, salt, and pepper.

Mash until smooth and creamy. Set aside.

Heat olive oil in a large skillet over medium heat. Add the onions, carrots, and celery. Sauté for about 5 minutes or until softened.

Stir in the minced garlic and cook for an additional minute.

Add the ground beef, breaking it up with a spoon. Cook until browned, about 8 minutes.

Mix in the tomato paste, beef broth, Worcestershire sauce, thyme, salt, and pepper. Bring to a simmer and cook for about 10 minutes until the mixture thickens.

Stir in the frozen peas and remove from heat.

Preheat your oven to 400°F (200°C).

In a greased baking dish, spread the meat filling evenly. Top with the mashed potatoes, spreading them out with a spatula to cover the meat completely.

Use a fork to create a decorative pattern on top of the mashed potatoes.

Bake in the preheated oven for about 25-30 minutes, or until the top is golden and the edges are bubbling.

Let it cool for a few minutes before serving.