1 1/2 tablespoons butter or margarine
1 medium onion, finely chopped
2 teaspoons all-purpose flour
1/3 teaspoon salt
1/4 teaspoon dried tarragon
1 pinch ground nutmeg
1 pinch cayenne pepper
1 (16 ounce) package frozen chopped spinach, thawed
2 cups chicken broth
3/4 cup plain yogurt
2 slices lemon, cut in half for garnish