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1
Heat the oven to 400 degrees.
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2
Peel one of the onions but leave it whole.
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3
Rub 1 tablespoon of the oil and a pinch each of the salt and pepper over the onion, then wrap it in foil. Roast the onion until it is softened and lightly caramelized, about 1 hour. Remove and cool.
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4
Meanwhile, peel and chop the remaining onion.
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5
Heat a medium, heavy-bottom saucepan over medium heat. Add the remaining olive oil and a pinch each of salt and pepper. Stir in the chopped onion, carrot and celery and cook until softened, about 8 to 10 minutes.
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6
Stir in the garlic, shallots and mushrooms and cook for 5 minutes, stirring frequently. Reserve one mushroom to garnish the finished soup.
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7
Stir in the wine, making sure to scrape the browned bits from the bottom of the pan. Stir in the fresh thyme and bring the contents to a gentle simmer. Cook until the mushrooms soften, about 5 to 6 minutes.
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8
Stir in the vegetable stock and continue to simmer until the liquid is reduced by half, about 10 minutes.
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9
Remove from the heat and puree the soup with an immersion blender, or in batches in a blender. Return the soup to low heat and stir in the cream. Season with a generous teaspoon of salt and a pinch of pepper, or to taste. This makes about 8 cups of soup.
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10
Chop the roasted onion into pieces no larger than one-quarter inch. Stir the chopped onion into the soup. Check and adjust the seasoning if desired.
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11
Slice the reserved mushroom into 8 very thin, lengthwise slices. Ladle the soup into warmed bowls. Garnish each portion with a thin shaving of the Parmigiano-Reggiano, one-fourth teaspoon of the chives and a thin slice of mushroom.
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12
Serve immediately.
