To prepare the tarte tatin:

1.Preheat the oven to 425°F.

2. Gently preheat a 10-inch cast iron or ovenproof skillet. Toast the walnuts in the pan, chop, and set aside.

3. Add the oil to the pan, wait a few seconds until it warms, then add sliced leeks and sauté until soft but not too browned. Remove the leeks from the heat and set aside.

4. Next add the mushrooms to your pan, season lightly with salt and sauté until they begin to sweat, about 5 to 7 minutes. Reduce the heat and add thyme, garlic, shallot, half the minced parsley and half the chopped walnuts. Sauté an additional 1 to 2 minutes, stirring regularly and shaking the pan. Season generously with salt and pepper and remove from the pan from the heat.

5. Spread the reserved leeks on top of the mushrooms right in the pan, then remove the pastry from the refrigerator and lay it over the leek/ mushroom mixture, gently pushing the edges of the pastry down around the inside edge of the pan.

6. Bake 20 to 25 minutes or until the pastry is puffed and a rich golden color. Combine the lemon juice and salt and whisk until the salt dissolves. Add the walnut oil, whisking again to combine. Taste for seasoning.

Remove the baked tarte from the oven. Run a knife around the outside edge of the pastry, set a serving platter over the skillet, and invert, gently lifting the pan away.

If any mushrooms stick to the pan, simply spoon them out and place them on top. Sprinkle with parsley and walnuts, drizzle with a bit of dressing, season to taste, and serve warm.