Preheat Your Oven: Start by preheating your oven to 375°F to ensure it’s ready for baking when you’re done preparing the dish.

Cook the Vegetables: In a large skillet, heat 2 tbsp of olive oil and 2 tbsp of butter over medium heat. Add the diced onion, sliced carrots, celery, quartered mushrooms, minced garlic, half of the kosher salt, and dried oregano. Cook everything together, stirring occasionally, until the vegetables are softened, which should take about 10 minutes.

Incorporate the Meat and Sauce: Stir in 3 tbsp of tomato paste and continue cooking for another 8 minutes, allowing the flavors to meld. Add the ground beef to the skillet, breaking it up as it cooks. Pour in ¼ cup of beef broth, the remaining kosher salt, ½ tsp of Worcestershire sauce, and a sprinkle of freshly ground black pepper. Cook the mixture until the beef is no longer pink, which should take about 3 minutes.

Prepare the Casserole Dish: Transfer the cooked meat and vegetable mixture to a 2-quart casserole dish, spreading it out evenly. Spoon 4 cups of mashed potatoes over the top, spreading them to cover the meat mixture completely, while leaving a small border around the edge of the dish. For extra flavor, you can sprinkle ¼ cup of grated Parmesan cheese on top, though this is optional.

Bake and Serve: Place the casserole dish in the preheated oven and bake for 40 minutes, or until the mashed potatoes are golden brown and the juices from the meat and vegetables are bubbling around the edges. Once baked, remove the dish from the oven and let it cool for 10 minutes before serving to allow the flavors to set.

Boil the Potatoes: Place the peeled and quartered russet potatoes in a large saucepan and cover them with cold water. Add 2 tsp of kosher salt to the water. Bring the water to a boil over high heat, then reduce to a simmer. Cook the potatoes until they are fork-tender, which should take about 15 minutes.

Heat the Milk and Butter: While the potatoes are boiling, in a small saucepan, gently warm ¾ cup of whole milk and 6 tbsp of unsalted butter over low heat until the butter is fully melted. This mixture should be warm, not hot.

Drain and Dry the Potatoes: Once the potatoes are tender, drain them in a colander. Return the drained potatoes to the saucepan and place it over low heat. Toss the potatoes gently for about 1 minute to evaporate any excess moisture.

Mash and Mix: Remove the saucepan from the heat. Pour the warm milk and butter mixture over the potatoes. Using a potato masher, mash the potatoes until they are slightly chunky, and not completely smooth. Stir in 2 tbsp of chopped fresh parsley for a fresh, vibrant flavor.

Season to Taste: Finally, taste the mashed potatoes and add additional salt and freshly ground black pepper as needed.