Place the sweet potatoes on a rimmed baking sheet. Pierce the potatoes a few times with the tip of knife and rub with 2 tablespoons olive oil. Roast, flipping halfway through, until the sweet potatoes are soft all the way through, about 1 hour. Remove from the oven and allow to sit for 5 minutes.
Cut the potatoes in half and fluff the inside with a fork. Drizzle each half with a tablespoon of olive oil. Sprinkle each half with a few coriander seeds, lemon zest and flake salt. Sprinkle with the parsley leaves to finish if desired.