The red grapes' sweetness brings out the nutty notes in the sprouts. Walnuts can be swapped for almonds or pecans.
Heat oven to 450 degrees. On 2 rimmed baking sheets, toss Brussels sprouts and grapes with oil and thyme. Season with salt and pepper. Roast, until caramelized and tender, about 20 minutes.
Drizzle each tray with 1 teaspoon vinegar and scrape up any caramelized bits with a wooden spoon. Toss in walnuts.