Curried-Squash and Red-Lentil Soup

INGREDIENTS

• 1 celery rib, chopped
• 3 tablespoons vegetable oil
• 1 1/2 pound butternut squash, peeled and cut into 1/2-inch pieces
• 2 quarts water
• 2 garlic cloves, minced
• 1/2 cup vegetable oil
• 1 carrot, chopped
• 1/2 cup chopped cilantro
• 1 teaspoon fresh lemon juice, or to taste
• 1 cup red lentils, picked over and rinsed
• 1 large onion, chopped
• Accompaniment: cooked basmati rice
• 2 tablespoons unsalted butter
• 1 tablespoon curry powder (preferably Madras)
• 2 tablespoons minced peeled ginger