Preheat oven to 350 degrees Fahrenheit.

Peel and coarsely chop the celery root.

Steam the celery root until tender, approximately 20 minutes.

In a food processor, combine the steamed celery root, 4 tablespoons of butter, sour cream, cream cheese, salt, and pepper.

Pulse and process until smooth.

Stir in 1/2 cup of sliced green onions.

Spoon the puree into a baking dish.

Dot the top of the casserole with the remaining 2 tablespoons of butter.

Sprinkle with the remaining green onions, grated parmesan cheese, and grated cheddar cheese.

Bake for 30 minutes, or until the cheeses are bubbly and lightly browned.

Serve hot.