Place leek in a 2-quart casserole coated with cooking spray; microwave at HIGH 2 minutes. Stir in asparagus and broth. Cover with casserole lid, and microwave at HIGH 10 to 11 minute or until asparagus is tender. Place flour in a bowl. Gradually add 1/2 cup water, stirring with a wire whisk until blended; add to asparagus mixture. Position knife blade in food processor bowl; add asparagus mixture, and process until smooth. Return mixture to dish; stir in tarragon and milk. Microwave, uncovered, at HIGH 6 to 7 minutes, stirring after 3 minutes. Serve warm or chilled; top with yogurt. Garnish with nutmeg, if desired. Yield: 4 servings (serving size: 1 cup soup and 1 tablespoon yogurt). Calories 147 (9% from fat), protein 10.7g, Fat 1.4g (Sat 0.4g, Mono 0.3 g, Poly 0.2g), Carb 24.1 g, Fiber 1.9 g, Chol 4 mg, iron 2 mg, sodium 138 mg, calcium 304 mg SOUP 70