Oven Roasted Tomatoes 8 barely ripe Romas. sliced ¼” thick (can be done with any tomato-roasting time will vary). lay them on a parchment lined sheet tray with a few sprigs of thyme, a light drizzle of olive oil, and a whisper of salt. Slow roast at 225* for about 4 hours or until shriveled around the edge. To store, cover with olive oil and place in the fridge.