Preheat the oven to 400°F.
Defrost the puff pastry slightly and unroll it onto a parchment-lined baking sheet. Use a paring knife to score a 1/4-inch border around the edge of the pastry. Make an egg wash by whisking one egg yolk with 1/2 teaspoon water, and brush the egg wash along the border. (You will not need all of the egg wash.) Return the puff pastry to the freezer until you’re ready to use it.
Slice the bacon into 3/8-inch-thick slices. Stack the slices in two piles, then cut the bacon crosswise into 3/8-inch even-sided rectangles or lardons.
Heat a large sauté pan over high heat for 2 minutes. Add 1 tablespoon olive oil, and allow to heat another minute. Add the bacon, and sauté over medium high heat 4 to 5 minutes, until slightly crisp but still tender. Reduce the heat to low, and toss in the young onions, thyme, and 1/2 teaspoon salt. Stir together a minute or two, until the onions are just wilted. Toss in the onion tops, and remove to a baking sheet or platter to cool.
Place the ricotta, remaining egg yolk, and remaining tablespoon olive oil in the bowl of a food processor. Purée until smooth, and transfer to a medium bowl. Gently fold in the crème fraîche and season with 1/8 teaspoon salt and a pinch of pepper.
Spread the ricotta mixture on the puff pastry within the scored border. Lay the Cantal over the ricotta, and arrange the bacon-onion mixture on top.
Bake the tart 20 to 25 minutes, rotating the baking sheet once, until the cheese is bubbling and the crust is golden brown. Lift up the edge of the tart and peek underneath to make sure the crust is cooked through. (If you underbake the tart, it will be soggy.).
Toss the herbs in a small bowl with salt, pepper, a drizzle of super-good olive oil, and a squeeze of lemon juice.
Let the tart cool a few minutes, and serve it on a cutting board at the table. Serve the herb salad in a small, pretty bowl.
To serve individual portions, cut six wedges from the tart and garnish each one with a little herb salad.