Step 1

Adjust oven rack to middle position and heat oven to 400 degrees. Combine garlic, 6 tablespoons oil, and 3 thyme sprigs in small ovensafe saucepan or 8-inch square baking dish. Cover; transfer to oven; and roast until garlic is tender and golden brown, about 45 minutes, stirring once halfway through roasting. Transfer saucepan to wire rack; discard thyme sprigs.

Step 2

Meanwhile, heat remaining 2 tablespoons oil in large saucepan over medium heat until shimmering. Add onion and ¾ teaspoon salt and cook until onion is softened and lightly browned, 10 to 14 minutes. Stir in broth, Parmesan rind, ¾ teaspoon pepper, and remaining 6 thyme sprigs and bring to simmer. Cover, reduce heat to low, and simmer for 20 minutes. Off heat, discard thyme sprigs and Parmesan rind.

Step 3

Using slotted spoon, transfer roasted garlic to broth mixture; reserve garlic oil in small saucepan. Stir ¾ cup bread cubes into broth mixture until thoroughly submerged. Working in batches (and filling blender about halfway in each batch), process soup in blender until very smooth, 1½ to 2 minutes per batch; transfer to large bowl. Return soup to now-empty pot and cover to keep warm. (Soup can be cooled and refrigerated for up to 2 days.)

Step 4

Line rimmed baking sheet with parchment paper and spray parchment lightly with vegetable oil spray. Toss remaining 4½ cups bread cubes with 3 tablespoons reserved garlic oil, remaining ¼ teaspoon salt, and remaining ¼ teaspoon pepper in large bowl. Add grated Parmesan and toss gently to combine.

Step 5

Arrange bread mixture in single, compact layer on prepared sheet. Bake until croutons are browned and crisp on bottoms and Parmesan has melted and turned golden, 12 to 16 minutes. Let cool for 5 minutes. (Croutons and remaining garlic oil can be stored in separate airtight containers at room temperature for up to 2 days.)

Step 6

Rewarm soup over low heat as needed. Serve, topping individual portions with croutons, reserved garlic oil, and dill.