Step 1: Brine the Peas and Carrots (Optional, but Recommended). Take half of your thawed peas and carrots (about 6 ounces) and place them in a small bowl. Pour the ¼ cup of dill pickle brine over them. Let them sit at room temperature for at least 15 minutes, or refrigerate for up to an hour. This will give them a subtle pickle flavor.

Step 2: Preheat Oven. Preheat your oven to 375 degrees F (190 degrees C).

Step 3: Cook the Lamb Filling. Heat the olive oil and butter in a Dutch oven (or large skillet with a lid) over medium heat. Add the ground lamb and diced onion. Cook, stirring occasionally, until the lamb is browned and crumbly, about 10 minutes. Break up the lamb with a spoon as it cooks.

Step 4: Add Flour and Aromatics. Stir in the flour until it’s evenly incorporated. Add the minced garlic, ketchup, rosemary, paprika, cinnamon, salt, and pepper. Cook and stir for 2-3 minutes, until the garlic is fragrant.

Step 5: Add Water and Simmer. Gradually stir in the water, scraping up any browned bits from the bottom of the Dutch oven. Bring the mixture to a simmer, reduce the heat to medium-low, and cook, stirring occasionally, until the sauce has thickened, about 5-6 minutes.

Step 6: Add Pickled Onions and Peas/Carrots. Remove the lamb mixture from the heat. Stir in the drained, pickle-brined peas and carrots, the remaining (non-brined) peas and carrots, and the finely chopped pickled red onions.

Step 7: Transfer to Baking Dish. Spread the lamb mixture evenly into the bottom of a 9×13-inch baking dish.

Step 8: Cook the Potatoes. While the lamb filling is simmering, place the peeled and halved Yukon Gold potatoes in a large pot of salted water. Bring to a boil, then reduce the heat to medium and cook until the potatoes are tender, about 15 minutes.

Step 9: Drain and Mash. Drain the potatoes well and return them to the pot.

Step 10: Add Remaining Potato Ingredients. Add the shredded Irish cheese, cream cheese, butter, and cayenne pepper to the potatoes. Mash until smooth and creamy. Season with salt and black pepper to taste.

Step 11: Enrich the Mash. In a small bowl, whisk together the egg yolk and milk. Stir this mixture into the mashed potatoes until well combined.

Step 12: Top the Filling. Spread the mashed potatoes evenly over the lamb filling in the baking dish, covering it completely.

Step 13: Bake. Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the sauce is bubbling up around the edges.

Step 14: Rest and Serve. Let the shepherd’s pie rest for a few minutes before serving. Serve hot, with thin dill pickle slices on the side.