Shepherd's pie with beef

Ingredients:

Casserole

• 2 tablespoons extra-virgin olive oil
• 3 tablespoons unsalted butter
• 1 medium onion, diced
• 3 medium carrots, peeled and sliced
• 1 stalk celery, chopped
• 220 g of champignons, cut into 4 pieces
• 2 cloves garlic, crushed
• 1 tbsp coarse salt
• 3/4 tsp dried oregano
• 3 tablespoons tomato paste
• 450 g of ground beef
• 1/4 cup canned lightly salted broth
• 0.5 tsp Worcestershire sauce
• 4 cups mashed potatoes, see recipe below
• 1/4 cup grated Parmesan (optional)

Mashed potatoes

• 0.7 kg potatoes, peeled and cut into 4 pieces
• 2 tsp coarse salt + more to taste
• 3/4 cup whole milk
• 6 tablespoons unsalted butter
• 2 tbsp. l. chopped parsley

Preparation:

1. Preheat oven to 190°C.
2. In a large skillet over medium heat, heat the olive oil and 2 tablespoons of butter. Add the onion, carrot, celery, mushrooms, garlic, half the salt, and oregano. Cook until the vegetables are soft, about 10 minutes. Add the tomato paste and cook until the mushrooms are soft and the tomato paste is browned, about 8 minutes more.
3. Stir in the ground beef, broth, remaining salt, Worcestershire sauce, and a pinch of black pepper, breaking up any chunks. Cook until the meat is completely browned, about 3 minutes.
4. Transfer the meat and vegetables to a 2-quart casserole and top with the mashed potatoes, leaving a 1/4-inch border. If desired, swirl the potatoes. Sprinkle with cheese, if using, and top with the remaining 1 tablespoon butter. Bake until the potato topping is golden brown and the juices are bubbling around the edges, about 40 minutes. Let cool for 10 minutes and serve.
5. Mashed potatoes:
   
   Place the potatoes in a saucepan and cover with cold water by about 2 cm (0.7 in). Add salt. Bring to a simmer over medium heat, uncovered, and cook until the potatoes are tender but not mushy, about 15 minutes. Meanwhile, combine the milk and butter in a small saucepan and heat over low heat until the butter melts. Remove from heat and cover to keep the milk warm.
   
6. Drain the potatoes and return them to the pan. Stir over medium heat until completely dry. Add the warmed milk mixture and mash with a potato masher until you have a chunky puree. Then stir in the parsley and season with salt and pepper to taste.