Pour a generous glug of olive oil into a large casserole pot or large saucepan on a medium heat.
Add the onion wedges and fry for 4 minutes, then add the whole mushrooms and fry for 10 minutes.
Preheat the oven to 180°C fan/200°C/gas 6.
Add the garlic, rosemary and bay leaves to the pot and fry for 1 minute. Then pour in the wine and simmer for 4 minutes.
Stir in the flour and cook for 30 seconds. Add the carrots, stock and dates, along with pinches of salt and pepper, and simmer for 2 minutes.
Transfer the mushroom filling to a pie dish.
Slice the potatoes into ½ cm thick slices and lay them on top of the filling, overlapping the slices as you go.
Finish with a drizzle of olive oil and sprinkle with more salt and pepper.
Roast for 40 minutes, or until the potatoes are tender and golden brown.