Put a baking sheet on the middle oven rack and preheat to 450 degrees F. Toss the Brussels sprouts with the olive oil, 1/2 teaspoon salt and a few grinds of pepper in a large bowl. Spread on the hot baking sheet and roast, tossing halfway through, until tender and charred in spots, 20 to 30 minutes.
Meanwhile, melt the butter in a large skillet over medium-high heat. Add the grapes and thyme and cook, tossing, until the grapes start to burst, about 4 minutes. Add the scallions and honey. Increase the heat to high and bring to a boil. Reduce the heat to medium low and simmer until the juices thicken and the grapes are glazed, 1 to 2 minutes. Season with salt and pepper. Transfer the Brussels sprouts to a serving dish; spoon the grapes over the top and sprinkle with the hazelnuts.