1.5 lb celery root with a knife peeled (sometimes called celeriac)
2 tablespoons parsley fresh chopped
0.5 lb haddock fillet smoked shredded frozen with your fingers into 1/2-inch pieces, discarding any tough parts thawed (finnan haddie)
2 cups heavy cream
2 medium onions halved lengthwise sliced lengthwise
2 lb potatoes peeled (preferably russet or Yukon Gold)
1 teaspoon salt
2 tablespoons butter unsalted
0.5 cup milk whole
Equipment
frying pan
oven
baking pan
aluminum foil
Directions
Preheat oven to 375°F.
Cook onions in butter in a 12-inch heavy skillet over moderate heat, stirring occasionally, until softened and just beginning to brown, 7 to 8 minutes.
Remove skillet from heat and stir in haddock, 1 tablespoon parsley, and salt and pepper to taste.
Halve or quarter celery root to fit slicer. Thinly slice potatoes and celery root (about 1/8-inch thick) with slicer.
Layer one third of potatoes and celery root in buttered baking dish.
Spread half of haddock mixture on top, then repeat layering with half of remaining vegetables, all of remaining haddock mixture, and then remaining vegetables. Stir together cream, milk, salt, and pepper and pour on top of gratin, then cover dish tightly with buttered foil.
Bake gratin in middle of oven until vegetables are just tender, about 1 hour. Uncover and bake, basting 2 or 3 times with pan juices, until top is golden, about 30 minutes more.
Let stand 10 minutes, then sprinkle with remaining tablespoon parsley.
