1.  Preheat the oven to 400 degrees.

2.  Defrost the puff pastry slightly then unroll or unfold it onto a parchment lined baking sheet.  Using a sharp paring knife, score a 1/4 inch border around the edge of the pastry.  Make an egg wash by whisking 1 of the egg yolks with 1/2 teaspoon water and brush the egg wash along the border.  You will not need all the egg wash.  Return the puff pastry to the freezer to set until you are ready.

3.  Heat a large sauté pan over high heat for 2 minutes.  Add 1 tablespoon of the olive oil and heat for 1 minute more.  Add the bacon and sauté over medium-high heat for 4 to 5 minutes, until slightly crisp but still tender.  Reduce the heat to low and toss in the Spring onions, thyme, and 1/2 teaspoon salt.  Stir together for 1 to 2 minutes, or until the onions are translucent.  Set aside to cool.

4.  In the bowl of a food processor add the ricotta, remaining egg yolk and olive oil.  Puree until smooth, then transfer to a medium bowl.  Gently fold in the crème fraîche and season to taste with salt and pepper.

5.  Remove the pastry from the freezer and spread the ricotta mixture within the scored border.  Lay the slices of cheese over the ricotta and top with the bacon-onion mixture.  (Note:  the tart can be assembled up to this point and refrigerated for up to 8 hours.)

6.  Bake the tart for 20 to 25 minutes, or until the cheese is bubbling and the crust is golden brown.  Peek under the crust to be sure that it is baked all the way through.

7.  Remove the tart from the oven and let cool for 5 minutes.  Cut tart into six wedges and serve warm or at room temperature alongside an herb salad.