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1
Toast the baguette slices and let them cool.
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2
Spread a generous layer of butter on each slice (you will need about 5 tablespoons), then lay the slices close together on a baking sheet and top with all but 1/2 cup of cheese.
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3
In a large saucepan, melt the remaining 4 tablespoons butter over medium heat.
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4
Add the onions, season with salt and saute, stirring occasionally, until very soft and golden, about 15 minutes.
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5
In a 5-quart casserole, arrange a layer of bread slices (about 1/3 of them).
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6
Spread 1/3 of the onions on top, followed by 1/3 of the tomato puree.
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7
Repeat for two more layers.
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8
Sprinkle with the remaining 1/2 cup cheese.
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9
To avoid boiling over, the casserole must not be more than 2/3 full.
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10
In a saucepan, bring 1 1/2 quarts water to a boil.
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11
Add the salt.
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12
Very slowly pour the salted water into the casserole, near the edge, so that the liquid rises just to the top layer of cheese without covering it.
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13
(Depending on the size of your casserole, you may need more or less water.)
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14
Preheat the oven to 350 degrees.
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15
Put the casserole on the stove and simmer uncovered for 30 minutes, then transfer to the oven and bake uncovered for 1 hour.
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16
The soup is ready when the surface looks like a crusty, golden cake and the inside is unctuous and so well blended that it is impossible to discern either cheese or onion.
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17
Each person is served some of the baked crust and some of the inside, which should be thick but not completely without liquid.
