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1
Peel the onions and cut in half through the stem ends.
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2
Rinse the halves in cold water for a moment (see box on facing page for why!).
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3
Slice the halves crosswise into very thin half-moon shapes; you should have over 12 cups of onions.
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4
Preheat the oven to 425.
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5
Put the oil and butter in the skillet, set over medium heat, and before the butter is all melted, dump in all the onions and turn them over to coat.
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6
Drop in the bay leaves, sprinkle with the salt, turn, and stir the onions a bit more, then cover the pan.
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7
Let the onions sweat and soften for about 10 minutes, stirring just once or twice, but otherwise keeping them covered.
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8
Uncover the pan, and continue to cook the onions and to evaporate the juices gradually.
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9
Adjust the heat to keep the juices bubbling without risking burning the onions, and stir frequently.
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10
After 10 minutes or so, the onions should be very soft, wet, and glassy, but with hardly any liquid left in the pan.
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11
If theres a lot of liquid, raise the heat slightly and cook longer, stirring as the juices evaporate.
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12
But dont let the onions or the pan get completely dry.
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13
Remove it from the heat while the onions are still moist, and let them cool off just a bit.
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14
Butter the baking sheet generously, using 3 to 4 tablespoons of the soft butter.
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15
Trim off the crusts of the bread slices, and lay the slices in one layer in the baking sheet, sides touching, to cover the bottom completely.
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16
Fill any empty spaces with pieces of bread cut to shape.
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17
Now press down gently over all the bread with your hand, to compress the layer slightly and close any gaps.
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18
Spread the remaining soft butter all over the bread with a rubber spatula or big spoon.
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19
While the onions are still warm, spoon them onto the bread and spread in an even layer; scrape the flavorful cooking juices from the pan too.
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20
Sprinkle all the cheese evenly over the onions, covering the entire surface, but dont press the cheese down at all.
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21
Bake for 10 minutes or so; rotate the pan for even cooking and bake another 10 to 15 minutes, until the cheese is a deep-brown-gold crust and the edges of the bread are also crisp and dark (but not burned).
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22
Let the gratin cool for at least a few minutes (the pan will be very hot!).
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23
Serve it warm or at room temperature, cut in pieces of any size.