Butter an 8 cup baking dish.

Preheat oven to 350°F.

Bring a large pot of salted water to a boil.

Thinly slice the green cabbage.

Cook the cabbage in the boiling water until tender, about 2 minutes.

Transfer the cabbage to a bowl using a slotted spoon.

Return the water to a boil.

Peel and coarsely chop the russet potatoes.

Add the potatoes to the boiling water.

Cook the potatoes until tender.

Drain the potatoes and return them to the pot.

Add butter to the potatoes.

Mash the potatoes and butter together.

Chop the chives.

Mix the chives into the mashed potatoes.

Add the cooked cabbage to the mashed potato mixture.

Season with salt and pepper to taste.

Spoon the mixture into the prepared baking dish.

Grate the extra-sharp cheddar cheese.

Sprinkle the cheese evenly over the top.

Bake for about 35 minutes, or until the cheese is bubbly and golden brown.