1. Adjust oven rack to lowest position, place large rimmed baking sheet on rack, and heat oven to 500°F. Toss fennel and mushrooms with 2 tablespoons oil, 1/2 teaspoon salt, 1/4 teaspoon pepper, and sugar in large bowl. Remove baking sheet from oven, and working quickly, carefully transfer fennel and mushrooms to sheet and spread in even layer. (Do not wash bowl.) Roast until vegetables are tender and well-browned on one side, 20 to 25 minutes (do not stir during roasting).

2. Meanwhile, whisk remaining tablespoon oil, lemon juice, mustard, 1/4 teaspoon salt, and 1/8 teaspoon pepper in now-empty bowl.

3. Toss hot vegetables with vinaigrette and cool to room temperature, about 30 minutes. Stir in radishes and reserved fennel fronds, transfer to serving platter, and serve.

Serves 4