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1
Dice the onion, carrot, celery and leek. I do this in the food processor pulse just until everything is chopped. In a large Dutch oven, saute the vegetables in 1/4 cup olive oil over medium-high heat until soft and tender, and the onion and leeks are translucent. Add the cognac and stir, scraping up and bits from the bottom of the pan, and cook until the cognac is evaporated. Add the stock, the chestnuts and the marjoram sprigs (count how many so you can take them out later). Bring to a boil, lower the heat, cover and simmer the soup for 45 minutes. Leave the soup to cool until it's safe to put in the blender. Meanwhile, prepare the marjoram oil (see below).
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2
Fish out the marjoram stems, then transfer the soup to a blender in batches and puree until smooth. After blending each batch, pour the soup through a wire mesh strainer set over a large bowl and push the soup through with a wooden spoon or spatula. There won't be much in the way of solids left behind, but straining the soup creates the velvety texture that makes this bisque so elegant. (For an even velvetier texture, you could push the soup through the strainer a second time). When you have strained all the soup, wipe out the Dutch oven and return the soup to the pot. At this point, you can cover the soup and refrigerate it for up to two days before completing it.
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3
When ready to serve the soup, heat it gently over medium heat, stirring occasionally, but do not let it boil. Slowly stir in the cream, incorporating it fully into the soup, then warm through. Serve immediately drizzled with marjoram oil.
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4
Heat the olive oil in a small saucepan just until bubbles appear on the surface and the oil is shimmering. Remove from the heat and leave to cool for two minutes, then drop in the marjoram sprigs, cover the pan and leave to cool. Strain the cooled oil into a jar or small spouted measuring cup for drizzling on the soup. The oil can be kept in an airtight jar for up to a week.
