Lentil Soup

Ingredients

• 2 russet (baking) potatoes
• 2 medium carrots, cut into 1/4-inch pieces
• 2 celery ribs, cut into 1/4-inch pieces
• 3 garlic cloves, chopped fine
• 1 medium onion, chopped fine
• 2 tablespoons unsalted butter
• 1 pound lentilles du Puy (French green lentils), picked over and rinsed (about 2 cups)
• 8 cups low-salt chicken broth