Heat oven to 400°F. Coat bread cubes in one-quarter cup olive oil and 1 teaspoon salt. Toast in a single layer on a half-sheet pan for 8-10 minutes. Flip the cubes using tongs and cook for another 8-10 minutes, or until just browned. Set aside to cool.

Increase oven temperature to 450°F. Drizzle 1-2 tablespoons olive oil over grape bunches and half the prepared thyme on a half-sheet pan. Sprinkle with 1 teaspoon salt. Roast for 15 minutes, flip each bunch and cook for another 15 minutes. Remove grapes and immediately deglaze pan with balsamic vinegar. Use a silicone spatula to scrape and mix juices on the pan. Pour deglazed mixture into a large liquid measuring cup with the remaining half-cup olive oil and 3/4 teaspoon salt. Whisk vigorously until a thick opaque balsamic dressing forms.

Layer together roasted grapes, prepared sourdough cubes, and fresh chèvre chunks on a serving dish. Drizzle balsamic dressing over top and garnish with loose thyme pieces. Sprinkle with flaky salt and freshly ground pepper to taste