Make the dressing directly in a storage jar the day before.
1 1/2 pounds mixed wild and cultivated mushrooms such as maitake, oyster, and king trumpet
1/2 cup extra-virgin olive oil, divided
3/4 teaspoon kosher salt, divided
1/4 teaspoon pepper
1 garlic clove
2 teaspoons whole-grain mustard
1 teaspoon honey
1/2 teaspoon fresh thyme leaves
1/4 cup Champagne vinegar
8 ounces celery root
12 cups loosely packed chicories, such as escarole and radicchio, leaves torn into bite-size pieces
How to Make It
Preheat oven to 375°. Wipe dirt off mushrooms, trim tough stems, and cut large mushrooms to make all pieces about the same size. Toss in a bowl with 1/4 cup oil, 1/2 tsp. salt, and the pepper. Spread on a rimmed baking sheet. Bake, stirring occasionally, until browned, 30 to 45 minutes. Let cool.
Make dressing: Sprinkle peeled garlic clove with remaining 1/4 tsp. salt, mince, and then flatten with side of chef's knife into a paste. Scrape paste into a jar with a tight-fitting lid and add mustard, honey, thyme, vinegar, and remaining 1/4 cup oil. Cap jar and shake until emulsified.
Peel celery root, then cut into matchsticks, dropping them into a bowl of water to prevent darkening. Pat dry, then put in a large salad bowl and add chicories and mushrooms. Toss gently with dressing.
Make ahead: Dressing, up to 1 day ahead, chilled. Prep vegetables 1 day ahead (chill celery root matchsticks in water), then dress just before serving.