Layered Potato Gratin with Ricotta and Feta

Steps

1. Preheat the oven to 400°F (200°C). Brush an 8x8-inch baking dish with one quarter of the olive oil.
2. Heat the remaining three quarters of the olive oil in a skillet over medium-high heat. Add the onion and cook 5 minutes until soft, then add the mushrooms and cook 6–7 minutes until browned and their liquid has cooked off.
3. Stir the garlic and thyme into the mushroom mixture, cook 1 minute, then take the skillet off the heat.
4. In a bowl, whisk the ricotta, the milk, the black pepper, the salt and two thirds of the feta into a smooth custard. Set aside the remaining one third of the feta for the top.
5. Layer one third of the potato slices in the dish, overlapping them. Spread half of the mushroom mixture and one third of the ricotta custard over them.
6. Repeat with another third of the potato slices, the remaining half of the mushroom mixture and another third of the custard.
7. Top with the last third of the potato slices and spread the remaining third of the custard over them.
8. Cover the dish with foil and bake 40 minutes, until a knife slides easily through the potatoes.
9. Uncover, scatter the reserved third of the feta over the top and bake 20 minutes more, until golden and bubbling at the edges.
10. Rest the gratin 10 minutes so it sets, then sprinkle with the parsley, cut into 4 squares and serve.