Corned Beef and Cabbage Shepherd's Pie

**Prepare the Corned Beef:** Place the corned beef in a large pot along with the onion, celery, and spice packet. Add cold water to cover. Bring to a boil over high heat, skimming off any foam that rises to the surface. Reduce heat to low, cover, and simmer gently until the corned beef is tender, about 3 1/2 hours. The internal temperature should reach at least 145°F (63°C).

**Cool the Beef:** Carefully remove the corned beef from the pot and place it in a bowl to cool. Taste the broth and add salt as needed.

**Blanch the Cabbage:** Bring the broth back to a boil over high heat. Add the cabbage and cook until it just begins to soften, about 5 minutes. Remove the cabbage with a slotted spoon and set aside in a bowl.

**Blanch the Carrots:** Add the carrots to the boiling broth and cook until they begin to soften, about 5 minutes. Remove the carrots with a slotted spoon and add them to the bowl with the cabbage.

**Cook the Potatoes:** Add the quartered potatoes to the broth and reduce the heat to medium. Cook until the potatoes are tender, about 15 to 20 minutes.

**Slice the Corned Beef:** While the potatoes are cooking, slice the cooled corned beef. Separate the brisket into two pieces along the fatty seam. Identify the direction of the meat fibers and slice against the grain into 1/4-inch thick slices, trimming off any large pieces of fat. Set the sliced corned beef aside.

**Preheat the Oven:** Preheat your oven to 400°F (200°C). Butter a 9x15-inch casserole dish.

**Prepare the Mashed Potatoes:** Remove the cooked potatoes from the broth using a slotted spoon and place them in a bowl. Reserve the broth. Add butter, salt, pepper, cayenne, and milk to the potatoes. Mash until very smooth. Stir in 3/4 cup of grated Cheddar cheese until well incorporated.

**Assemble the Shepherd's Pie:** Pour the blanched cabbage into the prepared casserole dish and press it down to create a compact layer. Distribute the blanched carrots evenly over the cabbage. Arrange the sliced corned beef on top, slightly overlapping the pieces and distributing the fatty pieces evenly. Pour 1 cup of the reserved broth over the corned beef. Dollop the mashed potatoes over the corned beef layer, then spread them into a uniform layer using the back of a spatula. Create a textured surface by dragging a fork sideways back and forth across the potatoes. Repeat this pattern from top to bottom. Sprinkle with a little more Cheddar cheese.

**Bake:** Bake in the preheated oven until the shepherd's pie is beautifully browned and piping hot, about 45 minutes to 1 hour.

**Rest:** Remove from the oven and let rest for 10 minutes before serving.

**Warm Remaining Broth:** While the shepherd's pie rests, warm any remaining broth over medium-low heat.

**Serve:** Cut the Shepherd's pie into squares and garnish with chopped scallions. Serve with the warmed broth.