Preheat oven to 180°C (350°F). Place the onions, cut-side up, in a greased baking dish. Drizzle with the oil and sprinkle with salt and pepper. Cover with aluminum foil and bake for 1 1/2 hours. Uncover and bake for a further 15 minutes or until just starting to brown. Combine the cream, thyme, mustard and cheeses and spoon over the onions. Bake for a further 15–20 minutes or until the onions are golden.