Trim both ends from each squash. Using a long, thin knife, scrape seeds from the cavity. Cut squash into ½-inch-thick rings, discarding any stray seeds. Combine squash, oil, thyme, salt and pepper in a large bowl, tossing to coat thoroughly. Spread squash in a single layer on a rimmed baking sheet. Drizzle with any oil mixture left behind in the bowl. Roast 20-25 minutes, or until tender, turning halfway through.