Make up the pastry using the rubbing-in method and chill for 15 minutes. Remove the chilled pastry from the fridge and use to line 25cm flan tin. Allow a little extra pastry to hang over the sides of the tin, and bake blind for 10 minutes at 200`c. Leave to cool before trimming the excess pastry off the edge of the flan case. Meanwhile, heat the oil in a large pan and fry the onion, garlic, bacon and thyme together for about 3 minutes till the onion is soft and the bacon is golden. Add in the sliced mushrooms and cook for a minute or 2 to remove any water, then throw in the spinach leaves and cook for 2 minutes to wilt. Remove any water from the pan and discard, then spoon this mixture onto the pastry case and even out. Sprinkle over the broken walnuts. In a bowl, mix together the ricotta cheese, eggs, cream and crumbled blue cheese and season with fresh black pepper. Spoon this mixture over the tart base and bake in the oven at 180`c, for about 30-35 minutes.