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Old-school French chefs are serious about sweating vegetables for soups, and you should be, too. Slowly cooking them in butter yields a gloriously creamy pureed soup—a potage—with nothing but water and stock, no cream, to adjust viscosity. We take sweating a touch further here, rounding out the almost citrusy aroma of celery root with chicken stock and thyme. Be patient and cook slowly over low heat. You're not looking to sauté the vegetables; that would leave some crunch, and you want them meltingly tender for the silkiest soup.

Both recipes on the next page are great as is, but either would also make a heck of a vegetable puree if you use just 2 cups of stock and no water.

Keith's Recipe Breakdown

Follow These Steps:• Melt butter in a Dutch oven over medium-low heat. Add celery root, onion, thyme, and garlic. Cover and cook 25 minutes, stirring occasionally.• Add stock, water, salt, and rind. Bring to a simmer; cover and cook at low 20 minutes or until very tender. Discard rind.• Add bread. Stir with a wooden spoon, scraping bottom of pan.• Pour half of soup in a blender. Remove center piece of blender lid (to allow steam to escape); secure lid on blender. Drape a clean towel over opening in lid (to avoid splatters). Blend until smooth; pour into a large bowl. Repeat with remaining soup. Return pureed soup to pan.• Stir in buttermilk; heat over medium-high heat until very hot (do not boil). Top soup with crème fraîche and chives.

CREAMY CELERY ROOT AND SWEET ONION SOUPHands-on: 20 min. Total: 1 hr. 20 min.

The main recipe is earthy and faintly sweet. The variation is surprisingly mellow; the garlic truly softens in flavor as it cooks.

SERVES 8 (serving size: 1 cup soup, ¾ teaspoon crème fraîche, and ¾ teaspoon chives)CALORIES 108; FAT 3.4g (sat 2g, mono 0.5g, poly 0.2g); PROTEIN 5g; CARB 15g; FIBER 2g; CHOL 8mg; IRON 1mg; SODIUM 293mg; CALC 73mg

VARIATION: CREAMY GARLIC SOUPSub 4 cups garlic cloves for celery root (omit the 8 cloves). Decrease onion to 2 cups; add 1 cup cubed peeled potato. Increase salt to ¾ teaspoon. Add 1½ tablespoons sherry at end; omit crème fraîche and chives. Top with a mixture of 1/3 cup toasted breadcrumbs, 2 tablespoons grated pecorino Romano, 1 tablespoon parsley, and 2 teaspoons lemon rind.

SERVES 12 (serving size: about ¾ cup soup and about 2 teaspoons topping)CALORIES 105; FAT 1.8g (sat 1.1g); SODIUM 219mg

For more cooking tips and kitchen science secrets, check out MAD DELICIOUS: The Science of Making Healthy Food Taste Amazing!