Preheat the broiler and position a rack in the middle of the oven.

Prepare individual casseroles or a large casserole dish on a baking sheet near the stove.

Combine potatoes and parsnips in a medium pot, cover with water, and bring to a boil over high heat.

Salt the water and cook until potatoes and parsnips are tender, approximately 15 minutes.

Drain the potatoes and parsnips and return them to the hot pot.

Add horseradish, milk, chives, salt, and pepper to the pot, and mash to the desired consistency.

Stir in 1 cup of cheese and the egg into the mashed potato mixture.

Heat a Dutch oven or large skillet over high heat with olive oil.

Add mushrooms and rosemary leaves and cook for 10 minutes.

Stir in the carrot and onion, seasoning with salt and pepper.

Cook for 5 minutes, then add garlic and kale, seasoning with nutmeg.

Cook until the kale wilts, about 2 minutes.

Stir in Worcestershire sauce and stock, and bring to a boil.

In a separate skillet, melt butter over low heat, whisk in flour, and cook for 1 minute to create a roux.

Stir the roux into the sauce to thicken it.

Transfer the vegetable mixture to the prepared individual dishes or a large casserole dish.

Top with the mashed potatoes, sprinkle with remaining cheese.

Broil for 2-3 minutes, or until golden brown.

Remove from the oven and serve immediately.