½ teaspoon freshly ground black pepper, plus more to taste
¼ cup heavy cream
Preheat the oven to 400°F. Halve and peel each onion, then slice each half into 8 thin wedges. Put the onions in a large bowl and toss with the thyme, oil, salt, and pepper.
Dump the mixture onto a foil-lined rimmed baking sheet and roast until the undersides are golden and a few edges are charred, 30 minutes. Remove the onions from the oven, give them a stir, season them with more salt and pepper and drizzle the cream over the onions.
Return them to the oven and roast until the cream is bubbling and clings to the onions, an additional 2 minutes. Remove from the oven, discard the thyme sprigs, and season with more salt and pepper to taste. Transfer to a bowl and serve.