Heat a very large, deep skillet over medium heat. Add all the spinach with the water still clinging to its leaves, even if you have to firmly pack it in. Cover the pan tightly (weight the lid down, if necessary). Leave the spinach to cook for about 2 minutes, then turn it over so that the wilted leaves are on the top. Cover and repeat the process 2 or 3 times more, until all the spinach is nicely wilted. Remove from the heat and cool to room temperature.
Squeeze the spinach dry with your hands a little at a time, squeezing as much liquid as possible from the leaves. Chop the spinach leaves coarsely and set aside.
In a medium saucepan, melt the butter over medium heat. Stir in the flour with a wooden spoon, to make a loose paste. Cook for about 2 minutes, until foamy, but do not let it brown. Remove from the heat. As soon as the paste has stopped foaming, whisk in the hot milk all at once and continue whisking until thoroughly blended. Return to the heat and stir slowly, reaching into all the corners of the pan, until the sauce comes to a simmer. Simmer for about 2 minutes, still stirring, until the sauce has thickened, then add the cream, salt, and pepper, and stir for 1 minute more. Remove the pan from the heat and stir it every minute or two while you finish the dish to prevent a skin from forming.
In the same skillet where the spinach was cooked, well dried, heat the olive oil over mediumlow heat. Add the onion and saute, stirring occasionally, for 7 to 8 minutes or until translucent. Add the cooked spinach and the white sauce to the pan and stir together over medium heat until well mixed and heated through. Finally, stir in the cheese, taste for seasoning, and transfer to a heated and covered serving dish.
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