Step 1

Heat 1 tablespoon oil in a large saucepan or pot over medium heat. Add leek and cook, stirring, until softened, about 5 minutes. Stir in garlic. Cook 1 minute more. Add broth, Jerusalem artichokes, potatoes and salt; bring to a boil. Reduce heat and simmer, covered, until the vegetables are very tender, 20 to 25 minutes.

Step 2

Puree soup with an immersion blender or in a regular blender. (Use caution when pureeing hot liquids.) Stir in half-and-half and thyme.

Step 3

Meanwhile, preheat oven to 425 degrees F. Place bread cubes on a baking sheet. Drizzle with the remaining 2 tablespoons oil, then sprinkle with garlic powder and pepper; toss to coat. Bake, stirring once halfway through, until browned and crisp, 15 to 20 minutes.

Step 4

Serve the soup topped with the croutons and thyme leaves, if desired.