Preheat oven to 450 degrees. On a rimmed baking sheet, toss kohlrabi with oil. Season with salt and pepper.
Roast until beginning to brown, 20 to 25 minutes. Stir; continue roasting until tender, 5 to 10 minutes more.
Meanwhile, melt butter in a small skillet over medium heat. Add hazelnuts and cook, stirring often, until golden, 3 to 4 minutes. Add thyme; cook until fragrant, 1 to 2 minutes. Stir in vinegar and maple syrup; cook 1 minute. Season lightly with salt. Transfer to a bowl. Add kohlrabi and toss to coat. Season with salt and pepper; serve.