Preheat the oven to 175°C/350°F.
In a non-stick frying pan, add the onion and extra virgin olive oil. Season with salt and pepper, and fry over medium heat for around 5 minutes. After this, add the garlic and fresh rosemary, frying for another couple of minutes.
Meanwhile, thinly slice your peeled potatoes, ensuring they are evenly sliced so they cook uniformly. Reserve about 3 large potatoes' worth of thinly sliced pieces for the top.
Add the remaining potato slices to the pan with the onion mixture and season generously with salt. Turn off the heat and stir until well combined. Transfer this mixture into a baking dish (I used a 9 inch x 12 inch dish)
Pour the chicken stock over the potato mixture in the baking dish, then layer the reserved potato slices on top. Drizzle with melted butter and season with salt and pepper.
Place in the oven and bake for 50 minutes, rotating the dish halfway through to ensure even cooking.
Once golden and fully cooked, remove from the oven and let it stand for 10 minutes. Optionally garnish with fresh parsley before serving.