Preparation

• Position a rack in the center of the oven and heat the oven to 350°F.

  In a 4- to 5-quart saucepan, melt 3 Tbs. of the butter over medium heat. Add the onion, leeks, garlic, 1/2 tsp. salt, and 1/4 tsp. pepper; cook until softened and light golden-brown, about 10 minutes.

  Add the broth, carrots, bay leaves, thyme sprigs, and 1/2 cup water; bring to a boil over medium-high heat. Reduce the heat to medium and simmer until the carrots are tender, about 15 minutes.

  Meanwhile, melt the remaining 3 Tbs. butter in a 3-quart saucepan over medium heat. Add the bread cubes and chopped chervil and toss to coat evenly. Spread on a rimmed baking sheet, season with salt, and bake until golden, 8 to 10 minutes.

  When the vegetables are tender, discard the bay leaves and thyme sprigs. With a regular or a hand blender, purée the soup (work in batches if using a regular blender). Stir in the yogurt. If you prefer a thinner texture, add a little water. Season to taste with salt and pepper, and serve garnished with the chervil croutons.