Put a pot or dutch oven over medium heat. Add Vegetable Oil (3 tablespoon) and allow it to get hot, then add Carrot (1/2 cup), Onion (1), Celery (1/2 cup), and Salt and Pepper (to taste). Stir and cook until the vegetable softened a little, about 5 minutes.
Add Garlic (3 clove) and cook for 30 seconds. Add Lean Ground Beef (1.5 pound), breaking it up with a spoon, add Dried Oregano (1 teaspoon), Dried Thyme (1/2 teaspoon), and Salt and Pepper (to taste). Cook until the meat has begun to brown, 5 to 7 minutes.
Add Tomato Paste (3 tablespoon) and stir until it's mixed with the meat. Add Chicken Stock (1 cup) and bring to a simmer, then reduce heat to low, cover and simmer for 10 to 15 minutes to allow it to thicken.
Stir and taste for seasoning. Stir in the Frozen Green Peas (1/2 cup) and set aside until needed.
Boil the Potato (2.5 pound) until soft and then mash with a potato masher until there are no lumps. Add Butter (1 stick), Milk (1/2 cup), and Salt and Pepper (to taste). Continue mashing to make potatoes smooth and creamy.
Spread meat filling evenly into a baking dish. Top with mashed potatoes, making sure to cover the meat mixture.
Place the dish on a baking sheet so the drippings won't dirty the oven and bake in a 400 degree F (200 degrees C) preheated oven for 25 minutes, or until potatoes begin to brown.
Cool for 15 minutes before serving. Enjoy!