Preheat the oven to 350 degrees F.
Combine the butter, vinegar, honey, thyme, salt, and pepper in a small saucepan over medium heat. Bring to a simmer and cook for 1 minute to reduce slightly. Place the onions, cut sides up, in a single layer on a baking pan. Drizzle the butter-vinegar mixture over and roast until soft and slightly caramelized, about 45 minutes.