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, skimming off and discarding any foam

Reduce heat to LOW, cover, and simmer gently until tender and meat thermometer reads at least 145 degrees, about 3 1/2 hours; remove to a bowl and let cool

Preheat the oven to 400 degrees

Butter a 9x15-inch casserole dish

Taste broth and add salt if needed

Bring broth to a boil over HIGH heat and cook cabbage just starts to soften and sweeten up, about 5 minutes; remove to a bowl

Add carrots to the boiling broth and cook until carrots start to soften, about 5 minutes; remove to a bowl

Add potatoes to the boiling broth, reduce heat to MEDIUM, and cook until tender, about 15 to 20 minutes; remove to a bowl and reserve broth

Mash the potatoes with butter, salt, pepper and cayenne until very smooth; then add 3/4 cup cheddar cheese

Pour cabbage into the prepared casserole dish and press on it until nicely compacted

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, then pattern with a fork

Sprinkle a little more cheddar cheese over the top

Bake until beautifully browned and piping hot, 45 minutes to 1 hour

Remove and let rest for 20 minutes

Warm any remaining broth

Garnish with scallions and serve with warm broth