Grease a shallow baking dish or heavy pan with a little oil. Cover the base with a single layer of onion rings from one and a half onions then follow this with a layer of potatoes using half the rounds.
Top the potato with two thirds of the cherry tomatoes, follow with 20 olives and sprinkle two thirds of the red pepper across the dish.
Cover with thyme leaves and crumble two thirds of the feta over the top. Season with salt and pepper and drizzle with two tablespoons of olive oil.
Cover these ingredients with another layer of potatoes using all the remaining rounds evenly spread. Drizzle the potato evenly with the remaining olive oil then finish the dish by using the remaining onion, tomatoes, olives, thyme and feta to finish the dish.
Pour the stock evenly around the sides of the dish and season.
Place your dish in the oven with a tight fitting lid or covered with baking paper and foil.
Cook for two hours, or until the potato is fork tender, before removing the lid to cook for a further half an hour or until the top browns. You can turn up the heat for a crispier finish if you chose.
Remove the gratin from the oven and allow it to sit for 10 minutes before serving.
TIP: Enjoy with leafy greens and balsamic dressing, BBQ meat or pan fried fish