• Add the potatoes to a large pot, fill it with enough water to cover the potatoes, and generously salt the water. Bring the potatoes to a boil and continue to boil until fork-tender, about 30 minutes.
• Meanwhile, make the meat filling. In an oven-safe medium braiser over medium-high heat, add the ground beef and lamb; cook, breaking it up to render the fat. Then add the diced onion, carrot, celery, and garlic. Season with 1 teaspoon salt and freshly cracked black pepper to taste. Cook, stirring occasionally, until just softened, about 5 minutes.
• Now, add the tomato paste and cook for 2 minutes until it has become a dark red color. Stir in the flour. Then deglaze the pan with the beef stock. Season with Worcestershire sauce and add in the herbs: thyme, rosemary, and bay leaf. Bring the meat filling to a simmer. Reduce the heat to medium-low and simmer the filling for 20 minutes.
• Taste for seasoning and add more salt if you think it needs it. Then stir in the frozen peas, turn off the heat, and remove the bay leaf and rosemary sprig.
• Preheat the oven to 400degreesF. Once potatoes are tender, drain them and run them through a potato ricer with the butter back into the pot. Now add the milk and salt. Gently mix the potatoes until everything is combined. Be careful not to overmix, the potatoes can become starchy and stringy.
I like to run the butter through the ricer with the hot potatoes, it allows the butter to melt into the potatoes without overmixing.

To Assemble the Shepherd’s Pie:

• In the braiser, spread the meat layer evenly. Then scoop the mashed potatoes evenly over the top. You can run a fork over the top to create ridges that will crisp up in the oven.
• Bake the shepherd's pie for 25 minutes until golden. Scoop the shepherd’s pie into bowls, top with chopped chives and enjoy!
For extra browning, pop it under the broiler for a few minutes. Just watch it closely!