All through the summer, I make a weekly trip on my bike to the Union Square Greenmarket. I always stop at Keith Stewart's stand to buy his amazing garlic, tomatoes, lettuces, and herbs. Just about every week, there are new offerings, so that I can gauge the progression of the season well into fall, when cool-weather vegetables like celery root and potatoes determine my cooking. Lately, I've been making Root Vegetable Crema, a velvety pureed soup that can be made with an endlessly variable mix of root vegetables such as potatoes, parsnips, celery root, and leeks. It makes a fine simple supper or satisfying first course for a dinner party.
This simple root vegetables soup achieves its extraordinary creaminess from a technique I developed in A New Way to Cook to concentrate the flavor of vegetables: braise the root vegetables slowly in water with a little butter. The small amount of fat suffuses the vegetables and gives them a rich, dense texture, delicious as is or enriched with a dollop of crème fraiche or whipped cream. The idea for this soup came from Paul Bertolli when he was chef at Chez Panisse in Berkeley, California. He used the suave puree as a medium for a fresh black truffle that he had pounded to a paste in a mortar to make an astonishing soup: truffle and root vegetable enhancing each other. The barest drizzle o white truffle oil is also divine.
This soup also takes well to garnishes plunked in the center of each bowl at the last minute, like crisp tiny dice of bacon, pancetta, or guanciale, or fine slivers of lardo or prosciutto.
If you have a fine slicing blade on your food processor, you use it to thinly slice the vegetables to save time.
• 2 teaspoons unsalted butter • 1 medium yellow or red waxy potato, thinly sliced (approximately 3/4 cup)
• 1 small celery root, finely diced (about 3/4 cup) • 1 medium leek, white part only, thinly sliced and rinsed (about 1/4 cup)
• 2 small parsnips, halved lengthwise, woody cores removed, and finely sliced (about 1/2 cup) • 2 garlic cloves, thinly sliced
• 1 sprig fresh thyme • 1/4 teaspoon salt
• 1/4 teaspoon sugar • 3 cups low-sodium or homemade chicken broth
• freshly ground white pepper • a few scrapings freshly grated nutmeg (optional)
• 1/4 cup heavy cream or crème fraiche, lightly whipped (optional)
Melt the butter in a medium saucepan. Add the potatoes, celery root, leek, parsnip, garlic, thyme, salt, sugar, and 2/3 cup water. Bring to a simmer, cover, and cook 15 minutes until the water has almost evaporated. Add the chicken broth, bring back to a simmer, cover, and cook an additional 15 minutes until the vegetables are soft. For the finest texture, puree the soup in batches in a blender. If using a food processor, let it run at least two minutes until perfectly smooth. Strain the soup into a saucepan. Adjust the seasoning. If desired, dollop or drizzle some cream into each serving.
The soup will keep three days covered in the refrigerator; it can be frozen up to two months.