Preheat oven to 400 degrees F with racks in the top third and lower third of the oven. Line a large rimmed baking sheet and a small rimmed baking sheet with parchment paper and set aside.

Brush olive oil lightly over both sides of each slice of bread and arrange in a single layer on the large baking sheet. Remove all stems from the grapes and toss with 1 tablespoon olive oil. Transfer to the smaller baking sheet and sprinkle with salt and pepper.

Place the grapes on the upper oven rack of the oven and the bread on the lower. Toast the bread about 10 minutes or until lightly golden and remove from the oven. Continue to roast the grapes for 10 more minutes or until slightly wrinkled.

To assemble, top each toast with enough brie, followed by grapes. Finish the crostini with a sprinkle of pepper and fresh thyme leaves.