For soup:
* 3 tablespoons vegetable oil
* 2 tablespoons unsalted butter
* 1 1/2 pound butternut squash, peeled and cut into 1/2-inch pieces
* 1 large onion, chopped
* 1 carrot, chopped
* 1 celery rib, chopped
* 2 garlic cloves, minced
* 2 tablespoons minced peeled ginger
* 1 tablespoon curry powder (preferably Madras)
* 1 cup red lentils, picked over and rinsed
* 2 quarts water
* 1 teaspoon fresh lemon juice, or to taste
For cilantro oil:
* 1/2 cup chopped cilantro
* 1/2 cup vegetable oil
* Accompaniment: cooked basmati rice
Make soup:
Heat oil with butter in a large heavy pot over medium heat until foam subsides, then cook squash, onion, carrot, celery, garlic, ginger, and 1 teaspoon salt, stirring occasionally, until vegetables are softened and beginning to brown, 15 to 20 minutes.
Stir in curry powder and 1/4 teaspoon pepper and cook, stirring frequently, 2 minutes.
Add lentils and water and simmer, covered, until lentils are tender, 25 to 40 minutes. Stir in lemon juice and season with salt and pepper.
Make cilantro oil:
Purée cilantro, oil, and 1/2 teaspoon salt in a blender.
Serve soup drizzled with cilantro oil.
