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1
In a soup pot or large Dutch oven, heat the butter and oil over medium-low heat.
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2
Add the leeks and 1/2 teaspoon salt and cook, stirring occasionally, until soft and translucent, 6 to 8 minutes.
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3
Add the potatoes, parsnips, celery root, and garlic, cover the pot, and stir occasionally until the veggies are heated through and softening a bit on the edges, about 10 minutes.
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4
Add the mustard, thyme, and bay leaf and continue to cook for 2 more minutes, or until aromatic.
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5
Add the wine and cook until the liquid has completely evaporated, about 1 minute.
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6
Add the broth and water, cover the pot partially, and increase the heat to medium-high.
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7
Bring just to a boil, then lower the heat to maintain a gentle simmer.
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8
Continue to cook, partially covered, until the vegetables are completely soft and have started to break down a bit, 45 minutes to 1 hour.
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9
To finish the soup, remove and discard the thyme stems and bay leaf and, working in batches, carefully puree the soup in a blender (or use an immersion blender).
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10
If you prefer a thinner soup, add a little more water to adjust the thickness.
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11
Stir in the cream and the lemon juice and taste.
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12
Adjust with more lemon juice or salt as needed and reheat as necessary.
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13
This soup is wonderful garnished with fresh croutons sauteed in butter and some chopped chives.
