Preheat oven to 400 degrees. Toss carrots, garlic, bay leaf, and thyme with oil on a rimmed baking sheet. Season with kosher salt and pepper. Roast, turning carrots occasionally, until tender and caramelized, 35 to 40 minutes. Remove from oven; let cool slightly.

Meanwhile, prepare an ice-water bath. Bring a medium pot of salted water to a boil. Add greens and cook until bright green and just tender, 1 minute. Transfer to ice-water bath; drain.

Remove garlic skins and mash cloves in a bowl. Whisk in yogurt and sumac. Season with salt and pepper. Let stand until sumac has started to tint yogurt, about 30 minutes.

Spread yogurt mixture on a serving platter. Top with sorghum, greens, and carrots. Drizzle with oil and season with flaky salt and pepper. Sprinkle with celery leaves. Serve immediately.