Corned Beef and Cabbage Shepherd's Pie

Put corned beef in a pot with onion and celery and sprinkle spice packet over top. Pour in cold water to cover. Turn heat to high and bring to a boil. Skim off and discard any foam if desired. Once boiling, reduce heat to low, cover, and simmer gently until tender, about 3 1/2 hours. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).

Carefully remove corned beef to a bowl and let cool. Taste broth and add salt if needed.

Bring broth to a boil over high heat. Add cabbage to the boiling broth. Cook until cabbage just starts to soften and sweeten up, about 5 minutes. Use a slotted spoon and remove cabbage to a bowl.

Add carrots to the boiling broth. Cook until carrots start to soften, about 5 minutes. Use a slotted spoon and remove carrots to a bowl.

Add potatoes to the boiling broth and reduce heat to medium. Cook until tender, 15 to 20 minutes.

While the potatoes are cooking, slice the cooled corned beef along the fatty seam to separate into 2 pieces. Identify the direction of the meat fibers and cut across them (against the grain) to slice into 1/4-inch thick slices, trimming off and discarding any giant pieces of fat. Set aside.

Preheat the oven to 400 degrees F (200 degrees C). Butter a 9x15-inch casserole dish.

Use a slotted spoon and remove cooked potatoes to a bowl. Reserve broth. Add butter, salt, pepper, cayenne, and milk to the potatoes. Mix and mash until very smooth. Toss in 3/4 cup Cheddar cheese and mix until incorporated.

Pour cabbage into the prepared casserole dish and press on it until nicely compacted. Evenly distribute and press carrots over the cabbage. Place a nice, even layer of corned beef on top, overlapping a bit and distributing the nice fatty pieces evenly until fully covered. Pour in 1 cup of the reserved broth. Dollop mashed potatoes over top, then spread into a uniform layer with the back of a spatula. Switch to a fork and push potatoes to the edge of the dish. Then drag the fork sideways back and forth over the potatoes to create a texture. Repeat this texture going top to bottom over the potatoes. Sprinkle with a little more Cheddar.

Bake in the center of the preheated oven until beautifully browned and piping hot, 45 minutes to 1 hour.

Remove from the oven and let rest for 10 minutes.

Meanwhile, warm any remaining broth over medium-low heat.

Cut Shepherd's pie into squares and garnish with scallions. Serve with warmed broth.