Preheat the oven to Gas Mark 5, 190ºC, fan 170ºC. Roll out the pastry on a floured surface to form a rectangle that will generously line a 28cm (11in) tin with a removable base. Press into the tin, leaving the excess overhanging. Cover the pastry with nonstick baking paper and fill the centre with baking beans. Bake for 10 minutes, remove the beans and cook for 10 minutes more.
Trim the excess pastry to the edge of the tin to create a neat finish and set aside. Leave the oven on.
In a large frying pan, fry the mushrooms in butter over a medium heat for 5 minutes. Add the spinach and the garlic and stir for 1-2 minutes to wilt the leaves. Spoon evenly into the pastry base.
Whisk the crème fraîche, milk (or cream), eggs and egg yolk together and season. Pour in the mushroom mixture and dot the Ripe & Ready French Brie slices evenly around the tart. Bake for 25 minutes, until golden and just set.
If serving the tart hot, rest for 5 minutes before slicing. Otherwise, leave until warm or at room temperature. Serve with the winter salad leaves.