Ingredients

• 1/2 cup minced onion
• 1 small rib of celery including the leaves, chopped fine
• 1 carrot, grated coarse
• 2 tablespoons unsalted butter
• 2 cups chicken broth
• a scant 1/4 teaspoon crumbled dried rosemary
• 2 1/2 cups mashed potatoes
• 1 teaspoon white-wine vinegar
• 1/3 cup minced fresh parsley leaves

Instructions

1. In a large saucepan cook the onion, the celery, and the carrot in the butter over moderately low heat, stirring, until the vegetables are tender, stir in the broth and the rosemary, and bring the mixture to a boil. Whisk in the potatoes, a little at a time, bring the soup to a boil, whisking, and stir in the vinegar, the parsley, and salt and pepper to taste.