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1
In a medium saucepan, cover the lentils with water and bring to a boil.
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2
Cover partially and cook over low heat for 20 minutes, stirring occasionally.
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3
Add 1/2 teaspoon of salt and cook until tender, about 5 minutes longer.
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4
Drain the lentils and spread them on a large baking sheet to cool.
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5
Preheat the oven to 350.
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6
Spread the hazelnuts in a pie plate and bake for 10 minutes, or until lightly toasted.
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7
Transfer the hazelnuts to a kitchen towel and let cool.
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8
Vigorously rub the nuts in the towel to remove the skins, then finely chop them.
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9
Squeeze the lemon juice into a large bowl of cold water and add the lemon.
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10
Peel the Jerusalem artichokes and drop them into the acidulated water.
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11
Using a mandoline, slice the Jerusalem artichokes crosswise 1/8 inch thick.
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12
In a large pot of boiling salted water, cook the slices until crisp-tender, about 8 minutes.
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13
Drain thoroughly and transfer to a large bowl.
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14
Add the lentils, hazelnuts, cream, lemon zest and garlic, season with salt and pepper; toss gently.
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15
Transfer to a 10-by-15-inch glass baking dish.
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16
In a medium bowl, toss the bread crumbs with the melted butter and scatter over the gratin.
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17
Bake for 30 minutes, or until the gratin is bubbling and golden brown.
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18
Let stand at room temperature for 15 minutes, then sprinkle with the parsley and serve.
