Preheat the oven to 190°C (170° fan) | 375°F | gas 5. Grease an oval baking dish with 1 tbsp of the softened butter.
Heat the olive oil in a large sauté pan set over a moderate heat. Add the garlic to the oil and sauté for 20 seconds before adding the mushrooms and a generous pinch of salt.
Sauté for 4-5 minutes, stirring and tossing from time to time, until the mushrooms are lightly colored and tender to the bite. Adjust seasoning to taste and set aside to cool.
Blanch the potato slices in a large saucepan of salted boiling water for 3 minutes. Drain well and leave to cool briefly.
Arrange the mushrooms in the base of the baking dish before topping with the potato. Generously dot the top with the remaining butter and season with salt and pepper.
Bake for 20-25 minutes until golden-brown on top. Remove from the oven and leave to stand for 5 minutes before serving with a garnish of chopped parsley on top. (Can substitute chives if you wish)
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