Heat the apple cider in a small saucepan set over medium-high heat and simmer until it’s reduced to about a quarter, and has the consistency of runny syrup.
Meanwhile, preheat the oven to 400°F.
Put the salmon or skin side down on a parchment-lined baking sheet, drizzle with olive oil, sprinkle with salt and pepper, and pull the leaves o of a few sprigs of thyme to sprinkle overtop.
Roast for 15–20 minutes, until the fat rises to the surface and the thin end flakes with a fork, but the fish is still moist in the middle.
Serve drizzled with the cider.