Instructions: Squeeze the juice of the lemon in a large bowl of cold water. Peel the cereliac and cut it into 1/4-inch-thick slices. To keep them from discoloring, drop the slices into the water as you work.

Drain the slices put them into a saucepan add the stock. Bring to boil and simmer over medium heat until the cereliac is tender, 10 to 12 minutes.

Keep the liquid.

Preheat the broiler.

To prepare the sauce, first heat the oil in a small saucepan over medium heat, add the scallions and cook them until they are soft. (2 3 minutes)

Whisk the flour, cook one minute. Whisk the milk. Spoon the liquid from the boiled cereliac and add it to the saucepan. Cook the mixture whisking constantly until it thickens, about one minute. Stir in the Gruyere, nutmeg, salt and pepper.

Place carefully the slices of cereliac in a gratin dish. Pour the sauce over them. Scatter the Parmesan over and sprinkle with paprika. Broil the dish until the top turns golden - about 2 minutes.

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