1. Preheat oven to 400 degrees. Put squash, celery root, onion, oil, and 1/4 teaspoon fennel seeds on a rimmed baking sheet. Season with salt and pepper; toss. Spread mixture out in a single layer. Roast, tossing occasionally, until vegetables are soft and golden brown, about 50 minutes.

2. Transfer mixture to a medium saucepan. Add wine and stock; bring to a boil. Reduce heat to medium-low; simmer, covered, 30 minutes. Remove from heat; uncover. Let cool 10 minutes.

3. Puree in a blender (never filling more than halfway). Pass through a sieve into a bowl; discard solids. Return puree to saucepan. Reheat over medium heat. Season with salt and pepper, if necessary.

4. Toast remaining teaspoon fennel seeds in a dry, small skillet over medium-low heat until fragrant and pale golden brown, 2 to 3 minutes. Let cool. Garnish each serving of soup with fennel seeds.