Begin by bringing a large pot of salted water to a boil. Add the peeled and diced potatoes, and cook until fork-tender, about 10-15 minutes.

While the potatoes are boiling, heat a large skillet over medium heat. Add pork lard or olive oil. Once hot, sauté the diced onions for 2-3 minutes until soft and fragrant. Add the ground beef, breaking it up as it cooks. Stir in the diced celery and carrots, cooking until the vegetables are tender and the meat is browned.

Stir in the tomato paste and Worcestershire sauce, ensuring everything is well coated. Let the mixture simmer for 5-10 minutes, allowing the flavors to meld together. For a thicker consistency, stir in a small amount of flour at this stage (optional). Remove the skillet from heat once the mixture is done.

Once the potatoes are done, drain them and return them to the pot. Mash thoroughly and mix in butter, milk or cream, along with your preferred seasonings (salt, pepper, garlic powder, etc.) until smooth and creamy.

Preheat your oven to 400°F (200°C). In a deep 9x9-inch baking dish, spread the meat mixture evenly across the bottom. Layer the mashed potatoes over the meat, smoothing the top for an even finish. For an extra touch, grate Parmesan cheese over the mashed potatoes before baking (optional but highly recommended).

Bake the Shepherd's Pie in the preheated oven for 20-25 minutes, or until the top is golden brown. Let it cool for 5-10 minutes before serving.